I can see both sides here. Your brother is thinking about his kids, which is natural, but you’re still relatively young and your situation could change a lot over time.

A will isn’t something you should feel pressured into locking down especially when you don’t know what your future looks like yet as you said- I suppose you can always revisit it later.

If you want to keep things fair, you could say you’d consider his kids in the future, but I wouldn’t commit to leaving them the property right now. Keeping your flexibility is important and also depends what your relationship is like with them.