I have to agree that I don't see the problem (posting to ask for clarification, not dogpile).
As far as I can tell, you talked to you 2xSIL, let's call her Veronica. Betty, your other SIL was also there. You told Betty and Veronica that you were considering separate birthday parties for your future children.
You are married to Veronica's brother, Joe. Veronica is married to your brother, Andrew.
Joe and Andrew were there when you talked about all of this, but they didn't hear you. Afterwards, Veronica spoke to Joe about the separate parties and expressed her disapproval.
Is that right?
If so, I think Veronica is allowed to talk to her brother about this because a)the hypothetical kids will still be her nieces and nephews, b)Joe is still her brother, and c)Joe was there anyway. Veronica could have just as easily expressed her disapproval with you there, but she waited for another time. As Joe's sister, I think that's okay for her to do.
Unless, of course, I'm missing something, which is entirely possible. I don't appreciate it when my SIL makes disparaging comments about my parenting plans, but my SIL tends to disapprove of a lot of what I do.