I wouldn't let your condition affect having a girlfriend. A relationship with someone will proceed as relationships do, regardless of what your chest looks like. Besides, there's no prerequisite for you to take your shirt off in front of her before starting up a relationship. Chances are, if she winds up liking you, she's not going to care about your chest anyway. And if she does, she's not worth having in the first place. Would you not like her if you found that her chest wasn't what you were expecting?
If you don't think your mom will understand, then I wouldn't tell her. Same for your step-dad. If they can't be supportive and help you and make you feel okay about your condition, then there's no reason to discuss it with them.
As far as swimming, I'd just wear a shirt into the pool. Yes, people will notice and they will ask you why you are wearing it. Depending on who they are, you can either choose to tell them the real reason, or not. If you tell them, I'd recommend being very matter of fact about it. The less concerned you are about it, the less they will be concerned, too.....and vice versa. Any resulting teasing will be a lot less because they aren't going to tease you as much about something they haven't seen.
Good luck with it.
