Thank you for the warning, I appreciate it also as a rape survivor..
There is a thing called 'secondary trauma', it's not uncommon for people who are close to rape survivors to experience a secondary pain, of course it is nothing like primary trauma but the way it affects people is very real (especially with how it can play out among these relationships too). I would actually urge you to join a support group (I'll PM you a link to a board if you like) where there is help for secondary survivors and primary survivors to talk out the pain. Since you have depression and from the sounds of it other life troubles anyway I would absolutely urge you to try to find therapy, a place where it's not selfish or bad for you to talk about ANYTHING, a time where it's solely dedicated to you. Where you're allowed to feel that pain without fear of treading on your friends' emotions, where you're allowed to vent out that grief and heal, as well as everything else.
Anyway yes it is true and real that the majority of rapes do not go even reported, let alone punished, for many many many many structural reasons such as what you are seeing, as well as police indifference/scorn/harassment, etc. For your friends to tell you about this is both a sign of great trust and probably overflowing emotionally; your emotions and expression of care would not be unwelcome, though from experience as a primary and secondary survivor I particularly urge caution around expressing violent thoughts or sudden anger that is not in support of theirs (a lot of people who are abuse survivors are also survivors of other types of violence / afraid of anger / additionally since there's incest involved here at an emotionally formative age the emotions are going to be complex). Also, try to avoid associating shame and disgust with what happened to them; any shame and disgust should be explicitly directed at the perpetrators.
The truth is you can't heal them, and they'll need support networks of their own, but by being there for them, believing them and being patient, affirming it's okay to talk and be grieving and not expecting them to be showing constant signs of progress and having 'gotten over it', understanding the hard time it's going to be giving them in all aspects of their life, and being a safe person to go to, you help be part of an environment where healing is possible. It depends entirely on your friends what type of comfort they want—it isn't a bad thing to ask what types of comfort they might especially prefer. Some people like hugs, some people don't, some people just want to be listened to, some people need affirmation and signs of support, validation, and belief. Some people need other forms of support like validating their anger (especially in huge cases of injustice like these).
As for the fact they're suicidal—realise that sometimes their best interests are not going to coincide with what they want, but be very, very careful of calling the cops on them unless you have very strong reason to believe they're goign to do something. It's a lot better to convince someone to seek help voluntarily at the ED than call the police (seriously I had the police called on me recently by my case worker because I said I wanted to talk about my suicidal feelings and they SHOWED UP IN MY ROOM WITH TASERS and caused me more problems, I ended up having the talk I needed to cheer up with a nurse in the hospital). For example I've been suicidal for a decade and only(!) attempted four times in all that. Hospitals make danger levels based on the following: having intention to do it, having a plan to do it, having the means to do it, and the detail/immediacy of the plan. Keep in contact with them though even if it's only to say you're checking up on them. Sometimes having someone to talk to is enough to placate those urges. Still, be mindful of those things though and other warning signs of increasing suicidal urge / intention, and if they might be concealing them from you.
And by looking after yourself, that's another way you can be there for them. I don't mean detaching from them exactly, but I mean finding a way to be okay and to still be their friend, for life to still go on, to not permanently dissociate from them or from having their lives changed. So it is important to look after yourself, and maintain your emotional space, spend time alone, find things to do and think about other than rape. Check up on your other friends. Some people find doing activism or attending events like Take Back the Night an uplifting way to deal with their emotions, some people just need to not think about rape and that's perfectly fine. If you find that it becomes an intrusive thought definitely definitely seek therapy. I would advise you of course not to talk about this to your other friends or anyone who isn't bound by confidentality / a rando on the internet unlikely to come across it I guess, which is why I strongly advocate therapy.
Most of all a lot of acceptance is required.. that a lot of violence happens in the world, that it's structural and individual alike, that it can be fought, that everything takes time and a lot of change, that it's not only your job to handle it, that everyone is interdependent on each other and requires love, care, and checking on (including you), that the future is arriving minute by minute and having even tiny short-term goals and things to look forward to are reasons to live... that such and such a thing happened to people you love but it didn't happen to you, that that's a good thing and you don't need to feel guilty about that, that you don't have to make all of it your pain but yes it's going to hurt when your friends hurt, it always will, and that's a function of caring and loving. Doing your bit proactively to help make the world a little safer for them and speaking out about the related issues is a way of fighting.
Hit me up if you need to talk :) I won't be offended if you need to talk secondary-to-secondary.
edit: While I wish Gabe hadn't brought up gamergate into this thread, I agree that opposing rape (of people of all genders, but it is disproportionately gendered) is a really important feminist issue, and being proactive about both learning and practising that is important. That being said, you don't need to think about rape 24/7 while you're still coping with this, but it's a positive motivator. Also agree with suggesting therapy in a nonconfrontational way when the timing is right but they probably have therapists... helping them stay in therapy (or change therapist if needed, etc.) is also a way you may be able to help them, don't know them personally, just pointing out a thing.
edit2: A thing I often say is that there's only a certain degree to which people's problems are relative. Everyone's worst problem is still their own worst problem. Please do not feel discouraged to get help for your own depression or like it does not matter (it is a thing your friends will understand and wish too).
edit3: Sorry I keep editing, just, this really hit me hard and I care a lot about this issue pretty much more than anything:
Also remember that this is a pain your friends have felt for a long, long time, all their lives; it is new to you, and it is being renewed all the time for them, and it sounds like some of them are at crisis points, but there's a reason we say 'survivor', even if often we do not feel like survivors, that we could have been barely said to be survived.
