Am I obligated to maintain ties of kinship with my wet nurse and my milk siblings, or am I sinful for cutting them off?

Maintaining ties with foster siblings is desirable but not obligatory. There is no sin in not maintaining these ties, but maintaining them is preferable. Maintaining ties of kinship is an act of closeness to and obedience of Allah, and its reward increases with the difficulty involved. The one who maintains ties is not the one who merely reciprocates, but rather the one who maintains ties when they have been severed. Therefore, maintain ties with your foster mother and foster siblings, as long as no harm or forbidden act results from it, and seek the reward from Allah without expecting any reciprocation.