I think my female cousin's shoulders seem broader with shorter hair
Does it bother you with how she looks?
Her shoulder seems bulky to me with shorter hair
That is not nice to talk about someone's looks. It is rude to insult someone's appearance. If she likes it then you shouldn't care whether or not it bothers you.
I think if she wants to go out of her way to make herself look younger, it is pointless. She should focus on herself instead.
I don't think you should get involved in how your cousin conducts herself. It is rude to insult someone's appearance especially if they didn't ask for your opinion. You should try and do things that make you feel better about yourself instead of focusing on other's flaws.