Story Poll – Which Plot?

I want to write a threesome story next where a husband and wife start out consoling a woman whose husband has left her for his mistress, but I can’t decide if she should be:

1. The wife’s best friend
2. Their sister-in-law (married to his brother or hers?)
3. Their son’s wife
4. The wife’s sister
5. The husband’s sister
6. Their daughter

Vote by comment?

Thanks.