LW said 'what's left of my relationship' with not-invited sister, so they obviously get along not so greatly, but LW seems to want to keep 'some' kind of contact with said sister, so perhaps is not so terrible to try to pacify her. OTOH, it doesn't sound like the sister and the daughter cared much for each other previous to this wedding thing, so it doesn't seem too terrible if sister decides to avoid daughter for a while, and the rest of the family lets them be, since both are adults and should resolve their issues without dragging everyone else with them?