LW, try this and see if you're still comfortable with it: substitute every implication of "sex work" in the letter with "acting" and see if it still feels like something you can stand behind. You discovered your stepsister had a profile on a theatre worker's web site (or heck, Model Mayhem, because a lot of actors also do modeling), and knew that the parents just wouldn't approve, so you told her off. And now you want to get in to the acting world - maybe you'd like to do a standup routine, or you think you can do some tech work for a community theatre and you'd like to have her introduce you to some leads.

See how that reads? That reads like you think acting is kind of a slimy thing, but you're not above using your family to make a buck. Sex work is professional work, and casting aspersions on someone's profession and then asking them for leads in that profession is either desperately slimy or actively harmful, especially when that profession has a troubled relationship with, for example, legal authorities who can use it as an excuse to ruin your life.