odds she's cheating? I don't expect her to tell me the "truth", who she met, etc. I have heard all the stories and am preparing myself for the worse, on the good side, I may end up booking one myself for me
Why would you assume something like that? Has she given you any reason to believe she would do such a thing? Odds are she is just having a nice time with her sis in law. Girl's go and have fun lots of times without being unfaithful.
There isn't anything wrong with taking a trip with your sister n law. This past summer I took a 7 day trip to Nashville, Tennessee with my mother, aunt, and sister n law. We had the best time. The thing is that my husband knows me, he knows that I am fully committed to our relationship, and he trusts me. It sounds like you all have a trust issue.
Well, if you are normally this suspicious, and then ultimately uncaring about your relationship, then chances are you have some problems in your marriage already.
It doesn't sound like you think highly of her at all, or value her much that would be the biggest problem that I can see.
As to the likely hood of her cheating, does she share your lack of interest in your relationship? Even then can't say, I don't know a thing about her.
My advice: Deal with your trust issues and decide if your marriage is actually important to you.