LW seems a bit confused on how to be a good house guest. Harassing her sister for a weekend every month is terrible manners. If LW knows in advance that her sister has pets that LW's daughter is allergic to, then a better response is to adjust her plans by renting a room in a pet-free hotel and interacting with her sister in a pet-free locations such as movie theaters, museums, and restaurants. This way, LW's daughter will not suffer from allergies.

It doesn't matter if pets are family. The pets have a home. LW is requesting that the pets be kicked out of their home for a weekend a month, every month.