I'm in a different situaton to you, in that one of the kids who lives with me is my nephew, his dad was my A (my brother)
We have a pretty good relationship, we talk about his dad, the alcoholism, the pain, bad days and days when we feel like just curling up and doing nothing.
I've personaly found it better to wait for him to tell me how he feels, I find if I tell him I'm having a bad day he tends to tiptoe round me and keep very quiet. A bad day to him still means a completely different thing than it does to me sometimes, he thinks bad days are his Dads bad days in drink. My bad days are tiredness and sadness missing my bro, and the stress of sorting things out.
Teenagers aren't the best at in depth conversations at the best of times, let them know they can talk to me, be honest with them and let them come to me is what I've done with mine (I've got 3) I'm not saying that's the right thing for everyone, but it's working for us right now.