My mom won't 'let' my younger sister go to the GYN....UPDATE

And my sister has been having irregular periods since she started her period ...she is now 19 and my mom doesn't want her to go because she thinks if the Dr puts her on the pill it will be ok for her to have sex. She is not having sex right now but has in the past. She goes like 4 months w/out a period! I want her to get checked out NOW because I am sure some day she will want to have children. Has anyone experience irregular periods like this and what was the cause? (If you don't mind me asking). FM me if it's better.

Thanks
UPDATE:
Spoke to Mom today....she brought it up and said my sister told her she spoke to me about it and that if my mom won't take her, she is going to planned parenthood. My mom sai d, fine I will take you but you are not going on the pill. My mom said she does not want her on the pill because she thinks my sister will think it's ok to have sex - I said to my mom, well then you need to have a serious discussion with her!!! I feel like the parent here. This is what happened with me, we never had any open & honest discussions about the birds & the bees! So, she is going to take her, and I said Mom - she needs to go. At least she will be regular, and have whatever it is taken care of now, and later on when she wants to have kids it won't be a mystery. My mom said Oh, she prob just has a hormonal imbalance - I was like HOW would you know that? You don't know that...she could have a cyst, she could have endometriosis - and you are just shrugging it off! I am SO glad I spoke up.