Request #659 from Barbara

I read this book mid—1980s. It may have been from the 1970s though. 

I can’t remember the MC’s name but she was the younger sister of a popular girl who was also a perfectionist. The MC borrows a blouse without her sister’s knowledge and stains it. MC has a boyfriend called Ryan. In the end both sisters are sort of envious of the other. 

I know this is pretty vague but I hope someone remembers it!