A small token. Heck, if you have a nice dollar store, sometimes they have very cute things there that don't look like they're a dollar (picture frames, vases, scented candles).
mini-ADD: I think it's sweet that you have your kids do it, but it's not necessary. But if it's something that you do for EVERYONE else, then not doing it for her may start something with your bro you won't want to deal with. If it's not a routine you do with everyone, you don't have to.
And I hate to say this, but your brother also split up his marriage. Unless you mean that you won't visit him if the woman is there, which is YOUR choice and is fine, then HE is responsible for you not seeing him because there is nothing stopping the two of you from getting together without the GF if you want to.