If you can take the details of most of this tread with a huge grain of salt, the overall message seems to be you need to stand up for your rights.
Since this is no longer in the heat of the moment, I suggest you sit down with someone who is supportive (hopefully your husband) and determine what is acceptable and what is not. Establish this line clearly for yourself first and determine the appropriate course of action for you to take should that line be crossed. Consider any consequences of your actions and make sure you can live with them. This does not have to be limited to SCUBA or this specific individual, but that is a good place to start. Your responses never need to be communicated, they are there to help you react with a well though out response should the need ever arise, rather than making that decision under stress.
The next step is to communicate what is and is not acceptable behavior to this person. Tell them what you want and how you expect them to behave. I would avoid using any "or else" statements, just make your expectations clear. Us men are notoriously bad at taking hints, so it needs to be communicated in no uncertain terms. For example, if my wife wants me to take out the trash, she will get much better results by saying "take out the trash" rather than "the trash needs to be taken out". I would tend to agree with her if she says the latter statement, but it won't necessarily occur to me that I need to take it out, much less now.
This conversation does not need to accuse, humiliate, or provoke confrontation with this person, just clarify what is and is not acceptable behavior. The objective is to get them to act in an appropriate manner around you, so treat them respectfully and as if they will meet all your expectations once you have communicated them. Any unpleasantness should be saved for your response should they ever disappoint you. Any responses should be appropriate to the offense, swift, with conviction, and with the goal of correcting the inappropriate behavior. I hope that you find a solution that enables you to maintain a good relationship with your sister, if not her entire family, while being treated respectfully.
-Chocula