B.,
You nailed it: The older son is sounding as if he is emotionally shattered by what has occurred in his life (loosing stability and who can blame him?)
What I would do:
While you have the two in care going thru this: Assure, Assure, Assure. Even if YOU may not be constant in their life (many people can come in and go out for whatever the reason), you are there for them now. Teens are not willing to talk, but let him know you are there if he does want to and that you will not tell his parents (unless it is life threatening, of course!).
You are a caretaker for them, and there is a bond there. Use it to the best of your ability.
For your own:
Try your best to explain the situation that the others are experiencing without going into much or going over their heads. Talk to them... I find even my four year old understands more than I realize when I talk with him (and he was that way when he was younger-) When we reason with our children, sometimes it does help a situation. It is also the "phrase of the hour". This is coming from a 'big kid' and big kids are so cool! in little one's eyes.
(Remember when you were younger and admired the older kids?)
I find if I try to avoid making a deal out of things, or if I say straight up "That is not nice, this is not how we talk to each other", it helps. (My son LOVES quoting his favorite shows, and sometimes other questionable things he hears!)
Most importantly: Patience... If you are frustrated and it shows, they pick it up and know it bugs you, so they keep it up. (something else I learned!).
Good luck and hang in there... :)