well, if it is because your mum finds herself without anything to do, why doesn't she do some voluntary work in an orphanage, or a hospital, if she needs that contact with children. Or she could do some other charity work, raising money to save other children from the fate of your sister. That way she could use the memory of your sister in a less creepy and more productive way. Just have a little chat with her, and if she doesn't change her mind about the party, then go and stay with your aunt as you mentioned before.
Sorry for your loss. xx