Family Life Predicament. Need advice

So this is gonna get complicated, fair warning. I am moving in with my b/f pretty soon. His parents aren’t charging me rent as long as I help do housework (they are semi-disabled). In the house also lives his older brother (we’ll call him A). A has recently went through a divorce and has partial custody over his 3 children (we will call them K, G, N). K G N are staying over in the house half the time. A kinda hoes around and has a girlfriend called S. S is also divorced and has three kids. A is not parenting KGN and is spending all his time with S. He has even gone as far as dumping off KGN with only his semi-disabled mother to take care of them (which she can’t). I feel terribly for KGN and want the best for them. I don’t want to step in and make A mad, but I feel as if KGN need an actual parent and I’m willing to fill the role. I really just want the best for those kids and I don’t know what to do about the situation. Any advice?