A Dilemma

Today I'm asking for input.  I have two daughters-in-law who hate each other and aren't subtle about it.  Now, I understand that two people don't have to like each other and just because you marry into a family, that doesn't make you kindred spirits with everyone in the family.  But, I can't understand not being civil at family gatherings and being polite.  I would be horrified if any of my children were rude at their in-laws home.  Basic manners are part of any group dynamic.

I'm not comfortable talking with either of them or my sons because I don't want to alienate any of them or fracture the relationship further.  Yet, if this continues, I can see it resulting in one or both of them refusing to attend any family function if SHE is there.

I love both my daughters-in-law and want them to feel like they are a part of the larger extended family.  I love my sons and want to see them support and encourage each other. I don't want my family to be divided.

Any suggestions?