I think you'll have to come up with the answer that best fits you, Soonbnude. Given the circumstances and presuming you really value the ongoing relationship with your sister in law, I think it may be worth honestly bringing it up again that you appreciate her dropping in and don't want your nude activities to get in the way of that. I'd be concerned she may be polite about it not bothering her while quietly deciding to not drop by as often. If she's genuinely comfortable with unannounced visits on side yielding a little surprise then all good, but if she baulks even a little at the prospect, then establish a little protocol (texting 'I'm on my way!', you automatically grabbing the shorts as soon as she arrives, etc.) to ensure nothing gets in the way of maintaining the relationship. Maintaining the effort to keep the relationship strong with your brother in law would also help.
Props for staying strong in your recent times.
I've have felt what the 'heck' even when the guest were nudist but didn't unclothe and were visiting for a while and I was the only naked. It got to be uncomfortable for me. I'm not to be a show or turn anyone in general I suppose is why I just felt out out of place. I am not a 'my place my rules' type. If Luann who is reserved about receiving guest naked no matter nudist friends or not without a covering they always follow her lead. I may not care for it but I will pull on some sweats in those times.