Alaskandude - I am truly sorry for your loss but far more sorrier for you and your situation (and actually your two sister's situation too)...
I do not want to pretend to understand but something truly bad must have gone down between your mother and father for things so drastic to have taken place.
In these situations, the innocent children are always the ones who suffer the most it seems.
I know that this is hard for you to hear right now but at the end of the day those guns were just 'stuff'. It may take you some time, but one can always buy and/or get more stuff.
The most horrible thing that I read in this entire thread was how your own mother kicked you out of a car and abandoned you in another state from your home.
This is unconscionable to me as a parent for another parent to do this. Unforgivable - dangerous - horrid behavior.
Since you say that filing suit against your mother would or could result in your young sister winding up in foster care - I say that this may be the best thing for her with a parent like this. How do you know that she won't just 'tire' of your two sisters and abandon them one day, as well. Also, it seems that if she did not have custody of the daughters that they would (at least after some time) go back to your father's care and not stay in foster care.
Regardless of any of that, you can know that one day you will or should get the last laugh. Most sociopathic parents who flippantly treat children like this will, as they grow old, one day try to mend fences as they get lonely. Generally, people like this (in your own words - fake and backstabbing types) end up alone and lonely. You of course, at that point, can decide how you want to react towards her and her sad excuses as a parent to you.
Good luck to you and I am sorry that you have been forced to learn this lesson about the nature of some people at your age - especially, someone who was recently so close to you.
Make sure that you remember this and how she made you feel and most importantly of all - that in remembering, you treat your children better if you ever have children of your own.
They (and you) deserve better!!!!