Answer:
The snacks are theirs.
Step-by-step explanation:
You said "my brother's". Let's think of this like groups. That means you and your brother are included into one group and his friends are in the other. If you and your brother are in a group, it wouldn't make sense to say they're his. It'd be theirs because his friends are in the other group and it's their candy.
I hope this helps you!