I do think the scenario is probably - kid at party is with dad and related to party people. His sister is not a child of dad, thus does not visit dad, thus would not go to the party. Party people i.e. OP and others, still thoughtfully send a bag of goodies because they know it can stink to have one sibling benefit from love, affection and gift from relatives that other sibling does not have.
OP, I think your choice to do this is kind and generous. It can not completely make up for the inequity that sometimes comes from siblings who have different families but it can sure help to reduce tension, jealousy, guilt and hurt feelings.