I am assuming that you supported your son that is got married for half the year and that her parents supported her for half the year. This would make it so you could claim your son and they could claim their daughter.

Two years ago I saved a tax client about $1,500 doing this. I think I did 3 returns for him (him, daughter and son-in-law) for about $300-400. I'm sure it's saved his in-laws a bunch of money too.

I learned this trick watching at tax partner do this with his son and daughter-in-law.