What are the limits of maintaining kinship ties, upon whom is it obligatory, and is a phone call sufficient when there is a great distance from a sister who is oppressive and does not like one's wife and children, knowing that her treatment was as God willed?

The jurists agree that parents, brothers, sisters, paternal uncles, paternal aunts, maternal uncles, and maternal aunts are the kin with whom maintaining ties is obligatory. You must maintain ties with your sister through visiting or calling, and the important thing is not to sever ties with her seeking Allah's pleasure. As for her injustice, address it with patience and kindness, and remember the Prophet's — may Allah's prayers and peace be upon him — saying: "If what you say is true, then you are feeding them hot ashes, and there will always be a supporter from Allah against them as long as you remain in that state."