Family wedding or Bachelorette (i’m the Moh)

100% agree. I assume you can't reschedule the bachelorette and also can't leave the bach for a few hours for the wedding and come back, as a compromise? If not, a few things to consider:

1. I would not say a wedding trumps prior commitments, especially if you're hosting and it's also a milestone celebration. A bach may not be a "milestone" in and of itself, but for me and many of my friends, having that quality time with the closest women in our lives was a hugely important rite of passage. If your friend feels similarly, I wouldn't discount that.

2. Most bach parties are a lot smaller than most weddings. I barely got to see most of my wedding guests, let alone spend any real time with them (we had ~100). If this is a typical size bach and 50+ person wedding, I can almost guarantee you will be missed far more at the bach. Especially given someone else would have to step in to host in your place, right? So multiple people could end up stressed by your pulling out of the bach party, whereas at the wedding you're only in a guest role (unless she's asking you to be in the wedding party, in which case her request has still come too late and it'd still be understandable for you to say no).

3. Think of it this way: would your cousin really expect you to bail on a bach party you already organized for your best friend just because she later scheduled her wedding for the same day (especially if the scheduling was "rushed" like you said)? Would you expect that of her? IMO, it'd be incredibly selfish and entitled for a bride to expect someone to bail on an intimate gathering they're hosting for their closest friend, just to be one of the many wedding guests she'll barely get to say "hi" to. I hope for your sake she is not like that!

4. FWIW, one entire side of my husband's family and one entire side of mine didn't make it to our wedding despite knowing the date for over a year in some cases. COVID was some of that, but some of it was other plans too. Nobody is holding it against them. It seems unreasonable that your family wouldn't understand if you didn't attend, especially if the date is sooner than expected. Do they really expect everyone in the family to drop all prior commitments anytime a family member announces their wedding date?

If you decide to attend the wedding, I would do everything you can to smoothly transition host responsibilities for the bach to someone else so you don't create stress for your friend - that seems important for preserving your relationship. But if it were me, I would 100% go to the bach and let your cousin know it was already scheduled, you're hosting and can't back out, but you'd love to spend some quality time with her and her new husband after the wedding and hear all about it / see pics and video. Probably a way better opportunity to connect and celebrate her marriage with her vs. actually attending the wedding!