A sibling-in-law is openly hostile to me. How should I respond?

Repel hostility with kindness, because answering bad with bad only prolongs the quarrel. Allah says: “Repel (evil) with what is better; then the one between whom and you there was enmity will become as though he were a close friend” (Surah Fussilat 41:34). Stay polite and avoid answering insults with insults. Keep a healthy distance if meetings spark arguments, without cutting family ties completely. Ask your spouse to deal with their family member. Do not spread the hostility to the children or other relatives. Pray for hard hearts to be softened, and leave the rest to Allah. See in-laws.