I'm sorry that they laugh about this. Nothing about it is funny and wrong. I'm not sure how your relationship with your mother is but if she's around you should try to talk to her about it. I wish all of these adults took this sort of thing more seriously. Trust your instinct. If you're that scared, go with your gut and stick to it. Don't let their carelessness impact your decision making.
I had a lot of people who I thought would protect me discard my instincts and made me feel like I was just paranoid. No one really noticed or understood. I'm sorry that that is also happening to you as well. That alone can get under your skin. I guess the best advice I can give you about this is to look after yourself and make sure that you're never without an adult in the same room with this person.