Is Hussein considered an uncle to the questioner, and is his wife forbidden to him if she is Hussein's sister from both parents, and would the marriage contract be invalidated, given that the questioner's paternal grandmother breastfed Hussein, the brother of the questioner's wife, and that the questioner's father is older than his paternal uncle Abdullah?

If Hussein was suckled by your grandmother five times within his first two years, then he is a brother to your father and your paternal uncles through rada'ah (suckling), and thus he would be your paternal uncle through rada'ah. As for your marriage to the sister of your paternal uncle through rada'ah, it is valid, because rada'ah does not affect the collateral relatives of the suckled child, and the prohibition (mahramiyyah) does not include them.