You mentioned your son plays rough with her, and she didn't come inside that one time. Is that rough play enough to keep her away, do you think?

Get her used to wearing just the harness when she is indoors. Then tie it to a leash and tie it to something outdoors. If there is something in the middle of the yard, tie it there - and if the leash is long enough, she can go wherever her sisters do.

She'll learn soon enough that going outside means using the harness - she will get used to it.

She'll be home soon.