All of those things cross massive boundaries into wildly inappropriate. I am so sorry you endured it.

1. Forcing children to be affectionate with adults used to be more common than it is now. It used to be considered misbehavior and/or lack of cooperation if they wouldn't willingly sit on Grandpa's lap, or go give Aunt Betty a kiss hello. Just because it was common doesn't mean it was right, or healthy.
2. No child of any age, including adults, needs to know about their parents' intimate activities in the bedroom. Children need sex education, yes, but not specific details about what their parents do and don't do behind closed doors. And they should be confiding their problems to a trained professional, not to their children.
3. Your mother's comments about your body changes would have been inappropriate even if you were alone with her. Even more so in front of anyone at all. Male cousin, older or younger, just makes it that much worse. And you're right about her being responsible for making sure you had a properly fitting bathing suit.
4. Making fun of your private body parts at 16. What is your mother's problem? That is utterly disgusting.

Again I am so sorry you had to deal with this.