Hi there, Rosa Carolina! Welcome to
my very own corner of planDisney! This is a great question, and I'm very glad you're checking before purchasing tickets.
Florida Resident ticket offers are intended specifically for Florida Residents. In other words, the benefit belongs to the person who qualifies for it and can prove Florida residency. Your sister-in-law would absolutely be able to purchase and use Florida Resident tickets for herself, provided she meets Disney's residency requirements and can present the appropriate documentation when requested.
However, that discount cannot be extended to friends or family members who are not Florida Residents themselves. Even if your sister-in-law is hosting your visit and purchasing the tickets, each person using a Florida Resident ticket must independently qualify for the offer.
Because Disney verifies eligibility for these promotions, it's very important to make sure everyone in your travel party is using the correct ticket type. I would hate for your family to arrive excited for a park day only to run into issues at the entrance because a ticket holder is unable to verify Florida residency.
The good news is that your sister-in-law can still help purchase
standard theme park tickets for everyone else if you'd like to keep your plans organized together. She just wouldn't be able to apply the Florida Resident discount to Guests who don't personally qualify for it.
Rosa Carolina, I know that's probably not the answer you were hoping for, but it's always best to know these details ahead of time. January is such a lovely time to visit
Walt Disney World, and I hope your family has an absolutely magical vacation together.
Warm hugs,
Ana Paula