What is the legal ruling regarding a husband reducing his contact with his sister — not cutting her off — to avoid problems she stirs up between him and his wife, and does any sin result from this, upon him or upon his wife?

There is no harm in the husband reducing his communication with his sister who is trying to cause discord between him and his wife, provided that he does not sever ties of kinship with her. It is preferable to overlook her actions, be patient with her, and repay her with kindness, so that Allah may transform her hatred into love, as evidenced by the verse: "And not equal are the good deed and the bad. Repel [evil] by that which is best, then abruptly your enemy will be as to you a devoted friend."