I'm going to make a few comments that I think are correct but I am not a lawyer so you should definitely consult one.
The first thing you have to understand is that when a property owner signs a quitclaim deed in favor of another, the owner is GIVING UP OWNERSHIP of the property. When your mother quitclaimed the property to your sister your mother was no longer the owner, your sister became the owner and your mother had nothing further to do with the property.
When your sister quitclaimed the property to you and her husband she GAVE UP OWNERSHIP. She no longer has any control over the property. She doesn't own it anymore. I can't emphasize that enough.
You and her husband are the owners. Full stop. She has nothing to say about the property.
That may have been required if she was selling her house to someone else and her husband might have been required to acknowledge that he was not claiming a marital interest in the home.
However, I don't think that's the case here because she GAVE him half ownership of the property along with GIVING you half ownership.
A word about ownership for a moment. In Florida when two unmarried people own property together the default presumption is "tenants in common" unless a different type of ownership is specified on the deed.
Important question: Is there a different type of ownership specified on the deed?
With tenants in common each owner has a 50% interest and can do what they want with their 50%. Theoretically, her husband can quitclaim his 50% back to her, and you and she would each own 50%. She, in turn, can quitclaim her 50% to her daughter and then you and her daughter would each own 50%
Your sister cannot compel you to GIVE her your 50% ownership.
I had a feeling you were going to say that. I'll bet you have put money into repairs, maintenance, upgrades, etc, which have seriously increased the market value of the property in the last 15 years.
You should be compensated for that. On the other hand, if you have been living rent free for 15 years, maybe not.
Well, there is a potential downside to all this. It's called a "Partition Action." Read about it at:
How an Action for Partition Works in Florida | DeLoach, Hofstra & Cavonis, P.A. (dhclaw.com)
You'd still have some leverage because the litigation is expensive, your sister would either have to have her husband file it or give his half interest to her, the property would have to be sold at auction where it would not get top dollar and, of course, the property would no longer be in the family. The risk to you is that you would no longer have a place to live and would have to rent or buy at today's prices. Which might not be too hard to take if you walk away with half the proceeds from the sale of the property.