I totally agree with other anon that you don’t have to ask permission. I would have HATED that. It isn’t thoughtful and it puts ME on the spot.

Also, this isn’t an acquaintance; this is family. She has actually provided care for the dog for a significant amount of time and also had a bond with him. She KNOWS her sister’s boundaries. She’s on here practically every Friday talking about visiting her sister or her sister visiting her. They’re close. It wouldn’t be remotely out of line to give her sister and brother in law a memorial gift for a soul they all loved and are grieving.