Would it be possible to frame it less as “I’m going with you to watch you” and more as “I’m going with you and we’re all going to have a great time”?
Obviously her care is a huge and ongoing undertaking, but this probably isn’t the last time she tries to find someone special, so maybe the ground rules are just the first meeting is a “double date” with you and your husband? Get in touch with the guy and go have dinner somewhere, if he’s just a creeper he’s not going to sit through dinner and probably will never show up. If he’s actually interested in getting to know your sister then your presence won’t seem odd at all, especially with your sister’s short term memory issues, and it’s just “meeting her family” for the guy.
I understand she won’t always remember your ground rules and I’m not pretending to understand the nuances of your situation. It does sound like she tends to check with you before going on these dates though, correct?
If you’re not able to make it, would she maybe have another friend who could fill the same role? Again, it’s not odd at all for a single woman (or anyone for that matter) to take a friend to a first meeting with someone, so neither side should feel like they’re an exception or being coddled.
If he is just an interested guy, that gives her the opportunity to find someone without accusing him of rape. If his intentions aren’t good, he’s unlikely to show up if he knows other folks will be around. If he does show up, you’ll have the opportunity to get a better read on the situation.
My cousin has some mental and physical issues and I never thought she’d find someone. Last year she got married to a nice man who likes having someone to talk to who doesn’t judge him and he seems to actually enjoy making her happy. Of course her family was skeptical throughout their relationship, but they seem to be good for each other and beyond that not much else matters. Maybe the same is possible for your sister.