The other day I was conversing with my SIL and other SIL when I said that when DH and I have kids rather then having a big party for both family's we may think about doing two seperate get togethers so to give each family equal amount of time with our children. When I said this it was just something that I was thinking about and certainly not concrete.
Issue: last night my DH say's to me "oh, by the way what's up with the whole seperate birthday party deal"? I was immediatly annoyed because for 1) DH was right there (but didn't hear me), 2) it certainly is not concrete and just something that I threw out there, and 3) not my SIL'S buisness to question DH regarding our marital decisions. Am I wrong for being annoyed? This is not the first time she's done something like this (i.e. talking about things that I say to my DH behind my back). I know the question will arise "what does DH think"? He's behind me and pretty much said when I showed annoyance "well, it's our marriage, and you know how she is" but seriously I'm super annoyed!
I'm still not entirely sure I understand what happened. I think part of what might be confusing the issue is that you're talking about two SILs! Would it be possible for you to give them fake names? Say your double-SIL is Jane and her sister is Susan?
As far as I can tell from what you've posted, though (this and your clarifications later), you mentioned something offhand to your SILs. One or both of them at least mentioned it to your DH. Then your DH asks you about it.
Perhaps I'm wrong, but it sounds like this is a situation where you shared a piece of information that you probably shouldn't have shared at that time. Saying something like, "We think we may do X," is very often interpreted by others as, "We've nearly decided to do X, but we want to gauge opinions before we actually say we're going to do it." So presumably, your SILs shared their opinions with you at the time, and then wanted to ask your husband about it.
But if you and your DH hadn't actually decided that was a firm possibility, then you really shouldn't have brought it up. I'll go even a step further and say that you brought it up prematurely no matter what you decided, because you do not yet have any children.
I guess what I'm saying is that you know your SIL is going to talk to your DH. It's not unreasonable for her to do this, as they are siblings. I don't think it's "dishing" unless she's doing it with an intent to make you feel bad. Some families spread information about each other in just exactly this way, and it's not inherently wrong. If you don't like it, the easiest and best way to stop it is to just not give them information you don't want everyone to know.