In Star Fluxx, how does the game end with Play all?
Your sister is correct. The Fluxx rules do say that "as soon as someone meets these conditions, they win!"

I don't think this should be an issue, it requires her to find the "play all" (or play 3, etc.) card and there are plenty of ways you can disrupt her strategy:


Change the rules so that there is a hand limit
Change the rules to increase the play limit
Use an action or surprise card to disrupt this strategy: Trash a new rule, Trade hands, rotate hands, rules reset, That's Mine! etc.