Urge adoption or butt out?

Ok, long story: my brother and sister-in-law have had custody of a child since he was a month old; he is now a year old. The mother is very young, immature and unwilling to be a mother to her children. (She also has a three year old that is being raised by someone else also.) She is now 21, I believe, and has cervical cancer. She was diagnosed after she had the first child. Obviously, the treatment didn't make her infertile because she had a second child, the one my brother and sister-in-law are raising.
The situation had me uneasy from the beginning because all the birth mother has to do is get a job and a place to live in order to regain custody of her children. (She voluntarily signed papers allowing them to be guardians of her children but did not legally sign away rights to them.) Now that I know about the cervical cancer I am even more worried. Of course, I hope the girl beats this thing and goes on to live a long and productive life but the aunt in me worries that if she becomes infertile (because of the chemo) that she will take the children back. My brother and sister-in-law act like they're in no hurry at all to legally adopt him. Should I say something or butt out. Or am I a total monster for worrying more about my 'nephew' than I do about this woman with cancer???