even though im a boy, i think i can help here.
im 17 and still going through puberty and at the stage of like a 12 year old.
so no your not alone in the way you do things etc. but having small boobs is not a bad thing. alot of boys prefer small boobs and also it even though its embarrsing dont let if affect you to much if you can. changing under your towel is fine, lots of ppl do that anyway. so your not alone and dont worry about it. and if you dont want to show your boobs to your cousin, thats normal just tell her and she should understand your privacy.