I suspect we’ve all known people like this — and you’re right; it’s difficult to be around them. That’s why often our first response is to ignore or avoid them if we can.
But God loves them just as much as he loves us, and he wants to help us love them also. After all, God doesn’t just love people who are easy to love; if he did, he’d turn his back on us every time we sinned. But God doesn’t do that, and neither should we. Jesus said, “If you love those who love you, what credit is that to you?” (Luke 6:32).
Ask God to help you see your brother-in-law the way God sees him, and to love him as you should. But love doesn’t mean we ignore a person’s faults or refuse to help them overcome them.
Pray for him, therefore, that God will show him just how prideful and insensitive he’s become, and how hurtful that is to those around him (especially your sister). Pray, too, that you will be patient with him, and that God will give you an opportunity to confront him about his attitude — not harshly but in love.
Pray most of all that he will see his need for Christ and humble himself at the foot of the cross. God wants to change him, and he will, as he opens his heart and life to Christ’s transforming power. The Bible says, “He guides the humble in what is right and teaches them his way” (Psalm 25:9).