R.W.
Have you Read the Toddlers Rules of Ownership?
We have the "whoever is touching it rule" here. If your Brother has the toy don't touch it, when he puts it down AND (very important and) walks away/starts playing with another toy. THEN you may play with it. You cannot follow him around waiting for the first opportunity to snatch the toy either. It takes a lot of GOYB (get off your butt :-) ) Parenting to gently redirect them and remind them of the rule but after hmmm I'd say 2 weeks or so for my kids the new rules sunk in and they now play quite nicely together and don't steal toys (Most of the time!)
When the rule is broken it's not necessary to punish either. Just make them give the toy back that's bad enough in their eyes LOL.