I was replying specifically to the comment that “people still have to clear the calendar that day, potentially travel for it, buy an outfit to wear, buy gifts, attend and probably plan a shower and/or bachelor party and bachelorette party.” Most of these things just don’t apply in this situation. If the SIL was planning a big affair I could totally understand, but again- with a family-only, 15 guest wedding, what is the likelihood that she’ll be having a large shower and/or bachelorette party to plan (and if she is, wouldn’t that be kind of weird/tacky?). For the size of these events, as described, 8 weeks apart seems fine.
You guys are giving the SIL the benefit of the doubt, but I have a feeling that she’s not gonna be happy with anything that puts her brother’s wedding before hers, no matter how early. ¯\_(ツ)_/¯