D.H.
Holidays is such a hard time to be sick. You want to see your family that you haven't seen in a while and boom...your kids get sick. I hear you though. She really shouldn't come and expose you guys to this. I'm one of those parents that wouldn't go over to someone's house if our kids are sick...just wouldn't do it. It's not fair to expose others to sickness. You are right in feeling that way. But, unfortunately you've voiced your opinion even though others didn't 'hear' it. Sadly, your husband should have told his sis the concern. My husband won't let anyone in this house if they are sick and he won't go over to someone else's house if they are sick and that includes his brothers that he only sees once a year. We have to set our priorities...and as you said, yours is your family/kids and it should be.
There is nothing else you can do except start building up the immune system of your own kids. I would give them vitamin C each and every day. Depending on their age/size. My 2yo, when she presents herself with a bit of a runny nose, I give her 2000mg daily and the runny nose stays away. There are other things out there to boost the immune system. We use Beta Glucan (you can get at a health food store). It's awesome stuff!
With RSV, your sis-in-law could give her child colloidal silver in a nebulizer. I've done this for pneumonia and it went away in just a day or two. Love it!
I hope all goes well.