Adoption Support Group
Adoption is the legal act of permanently placing a child with a parent or parents other than the birth parents. Whether you are looking to adopt, putting your child up for adoption, parenting an adopted child, or just thinking about it, this is the place to discuss.
You have to protect them now. Should something happen to your husband, custody will automatically revert to her. They will be taken from you and their home. Is she capable of caring for them? You have to consider what is best for them. If that means adopting them, I encourage you to teach them to love and respect her. Keep healthy contact with her as much as possible. (Even if the kids can't have contact. You should, if you can.) It will make it easier for you and her (and one day, for the kids).
If you have to do this, you are not violating her, you are doing her a service, by caring for her children when she cannot. No child has ever suffered from being loved too much. The only violation would be if you degraded her or her memory to them. NEVER, NEVER, NEVER bad mouth a birth parent in front of an adopted child.
Children are for the rest of your life, not just when you feel like it. It sounds like she has chosen her path.