In that case I don’t think you should go. Your son is 14, he will be able to tell that there is bad blood. Maybe sit him down and explain that you would like to go for his sake but think it’s too soon and you are not comfortable around them yet (without throwing blame on them, as difficult as that may be).
Only knowing your cup size is not enough. You need to know the underbust size as well. A 32D and a 34C have cups with the same volume. Sure, there is still some variance but not as much as I thought before I learned that.
Edit: This calculator and the community of the same name on the-site-that-shall-not-be-named helped me a lot in finding my actual bra size. Now my only problem is that almost no company here has more than two or three bras in that size…