My H has also taken on other people's kids. For the past nine months or so, he's felt bad for a friend whose husband left and pretty much ignored his three daughters. So H spends time and money on them so they don't feel left out.
Now, my H had custody of his S11 for about six weeks this summer and didn't spend much time with him at all. He was busy taking on jobs and even having his bf watch S so that he could go out with other friends.
I think our H's do want to be there for their kids, but they just can't be right now. They think they can be good influences on the other kids, but are just reminded of their failures with their own.
I know it's frustrating, but you can't do anything about his actions. Just be the best mom you can be as that is all you can control.
M36 XH34 M-5 T7 4/11 H confused 5/11 ILYB 6/11 OW discovered 7/11 I move out, OW over 5/12-OW2,done->new EA, but H wont file 9/12 H "best bf ever" to EA/OW3 3/13 H/OW break up H files 4/13 D 6/18/13