Canceling bachelorette party

I'm sorry you're going through this! If you decide to cancel, the cancellation should come from whoever extended the invitations to the guests in the first place. It sounds like that person may be your sister. She could say something like "Unfortunately, the bachelorette party on MM/DD/YYYY needs to be cancelled, but we can't wait to celebrate with you at the wedding!"

This is 100% on point if you decide to cancel.

My recommendation would be to scale back the bachelorette so you can still have it. Coordinating many different people, planning weekend full of activities, and providing accommodations, food, decorations, etc for everyone is a lot of work. Doing a night out, a day trip, or 1-2 fun casual activities is not. If what is currently planned is too much work for one person and no one else is willing to help out, scale back so that the amount of work is appropriate for one person and you can still have a fun bachelorette.

Also, there is no reason why other people cannot help with your bachelorette if they want. Brides with no or small bridal parties shouldn't automatically be denied a bachelorette just because there are fewer bridesmaids to distribute the work and cost among. It would, IMO, not be inappropriate for your sister to reach out to some other ladies on your bachelorette list whom you are close to to ask if they might help in one way or another. One person can be in charge of decor, another can plan food, one can do activities, etc.

If I were asked to help do this for a friend, I absolutely would, but I never want to overstep so if there is an MOH or whatever I always assume they are in charge and defer to them, and if nothing happens I assume that's because its what the bride wants, not because there were a lack of people to chip in and help.

If you end up cancelling I think Allie's wording is perfect!

I'd also see if you can do anything to ease the planning stress on your sister - would any of the other bridesmaids be willing to help plan!