My mom adopted my niece & now she is on her death bed. In her will, she appointed my sister to be her guardian. The rest of the family doesn't feel that's the best choice. Can it be contested?

Can't you all put aside your petty squabbles and enjoy the little time you have left with your loved one? There will be plenty of time after she's gone for you all not to get along. Let your neice have her peace. She appointed your sister for a reason.... Unless your sister put a gun to her head forcing her to appoint her. If that's the case then a call to the police is suffice.

Anything in a will can be contested.

You mention your mom and your niece and than say "she" is on her death bed.

Who's the one dying?

Wills are where the executor is named.l

Guardians are appointed before death, not in the will.

If you truly understand what you are talking about, you have yeet to communicate what you understand accuratelyu to us.