What do you do when you in love with your sister-in-law?

This is a situation that is so painful for other members of the family too, as you have found out. From my own experiences, support the cheated partner as much as possible. They are your friend quite apart from the relationship by marriage. You may feel personally cheated out of being officially family with them should a divorce occur and feel you have no control over that happening. Somehow helping them out practically and emotionally helps you feel that you can make a difference and conveys the message that they really matter to you...even if it is your brother that has broken their heart.

Often the person who left will avoid you because they can't face you. Write a letter expressing honestly how wounded you feel, but that you are still there if they need you and leave it at that. Be honest about how you feel but keep your cool. You might like to point out that you are supporting your sister-in-law as you care about her a great deal. Make absolutely sure you keep in strict confidence whatever either side confides to you and don't become a go-between or become otherwise involved in any conflict...leave that to a counselor or minister.

The worst times are at Christmas etc. if you have to face the new couple. I wish I had a good suggestion for how to handle this easily as it an be very uncomfortable for all. Perhaps speak with your brother beforehand to tell him that it is going to be difficult and ask how he is feeling about it. Alternatively, you could make a statement by spending the day with your sister in law...it will be a worse time for her than for anyone else. If there are children involved, go wherever they are and make it special for them.