My sister in law recently passed away, her biological father and his wife (not the birth mother) have taken custody of 3 children. I believe they are planning to adopt. If my father in law passes away, does his wife keep the children or can her brother and I have guardianship?

If THEY adopt the children they are the legal parents. If one passes away, the other "keeps" the children.

If one biological parent passes away the other doesn't give up the children.

If you want guardianship, ask that that provision be made in their Wills.

Personally is love for the children is their main concern then I guess that makes your question pretty futile