I wouldn't take in my nieces or nephews, I love them with all of my heart and that's why I wouldn't take them in. I'm bi-polar, so is my husband, we both have a history of violence when stress (even on medication) and have both spent time in a mental hospital. I love them enough to know the harm I could do. My own parents were verbally, emotionally and physically abusive to me, I know the damage it can cause. I don't know how else to discipline without using violence, I also have extreme difficulty expressing emotions, especially affection or love. My niece and nephews would be better off in foster care with the hopes of adoption by a mentally normal couple then being raised by me.
Only exception would be if they were older teenagers were they could mostly take care of themselves or fight back if I got violent. I think then I could handle it better, nothing else I would pay for them to have an apartment nearby if thing got to bad with me. My brother came out (he's gay) when he was seventeen and did okay living by himself after my parents kicked him out. Lucky with my brood of siblings that would never be a problem. While we all suffer for a childhood of abuse and the lack of affection from our parents, most of them do not have the bi-polar/violence problems that I do. Nobody married anyone that is an only child except me, so there is family on the other side that could also step in. My parents are still alive but I doubt any court would grant them custody, long record of CPS being call in for abuse.
If it was me with the kids, I would respect their decision, not everyone is cut for kids. I find it hard to believe that a person wouldn't have a trusted friend or relative like a grand-aunt/uncle to children that couldn't care for them. The parent made the decision to have the child, not the sibling, it's not their responsibility to care for the children. Money is not everything, some people like their freedom, career, or simply enjoying quiet time without the distraction of children. Like I said, it may seem harsh, but the siblings did not make the decision to have that child. Why should they be strong-armed into doing something they never wanted in the first place, simply due to circumstace.