Sister probably said to do it. He's dead, so it would be up to her. Even if he has a will laying around somewhere, it wouldn't necessarily spell out what was to be done with his body.
Wasn't James Brown's body sitting in a freezer for damn near a year before they decided to put him to rest, due to issues with his estate? They cremated Prince fast.
Sure, however, if he had left instructions regarding what was to be done to his body it suggests he may have other
instructions regarding his death too, i.e his estate etc.