Is it impermissible to avoid communicating with one's sister due to incitement from the stepmother, out of fear of misunderstanding or escalating problems, with the intention of communicating later?

You must maintain ties with your sister, for she is among the kin with whom ties must be maintained and severing them is forbidden. Maintaining ties of kinship is one of the greatest acts of devotion, and severing them is one of the gravest major sins. The one who maintains ties is not the one who reciprocates, but rather the one who maintains ties with his kin even if they sever them. Demanding one's rights through legitimate means does not contradict maintaining ties of kinship. Continue to maintain ties with your sister and your stepmother, and be kind to them, for that dispels animosity. If you fear a definite harm, you may postpone contact while striving for reconciliation.