Regardless of if you are an asshole or not, It's ok to be an asshole.
Your first responsibility is to your immediate family. I know I wouldn't take in some messed up kids if it would compromise the quality of life for my own children and I wouldn't feel bad about it. Quality of life, paying for college, paying for weddings, paying for cars, other things a parent does for their kid. If any of those had to be compromised because you took on additional kids, that's not right or fair to your own kids. If you have to forfeit your retirement funds to pay for them, that's not right or fair to you and your wife.
I have two nephews and they're good kids and my brother and SIL are good people. If something happened to them I would take in my nephews, no different than they'd take in my kids if something happened to my wife and myself. That being said, we have kids the same age and we both have money that would go to supporting the kids so it wouldn't be a financial issue and I wouldn't be compromising my retirement years to be a parent.
I wouldn't take on small children, I wouldn't take on children that don't have any other financial support, and I wouldn't do anything to help a shitbag who has put themselves in a bad situation through their own poor life decisions.
Ride 'Em Cowboys!