Upon whom is the maintaining of kinship ties (silat al-rahim) obligatory: upon the maternal uncles and the sister's children, or upon one of the two parties?

Maintaining kinship ties is obligatory for everyone. Both the maternal uncle and the sister's children are commanded to uphold it. It is not optional, nor is it based on the principle of reciprocation. The religious texts related to maintaining kinship ties are general and comprehensive, encompassing everyone.