I agree with Anna. I don't think you need to change your plans to suit someone else. Who cares if you steal her thunder?? What if you had caught a couple months ago, do you think she would have put her wedding on hold so as to not steal your thunder? I highly doubt it!! I think you guys should continue TTC if that's what you two want, and don't worry about anyone else. Personally, I think it doesn't really matter what MIL wants anyway, as she won't be the one raising #3. There's no reason why SIL can't get married and you guys have a newborn and all of the family be celebrating for both of you at the same time!! Sorry if this sounded harsh, just my two cents! Good luck hun!