Guys please pay your attention on heavy bleeding, Take this matter seriously and go for VOG. Most of the time they will tell it's normal but please ask them to diagnose and prove it. This may be symptoms of endometriosis. If you act in early stage you can have babies, otherwise you she may not able to give a birth. If you married this it simple thing to go to a VOG and check. Go for at least three VOG's to confirm the case before you going for a surgery. Moreover, if she is your sister then you need to suppress her cycle until she gets marry. I am not a doctor but i know things about the case. This is serious bro!
https://www.nhs.uk/conditions/endometriosis/
Be educate yourself and give care to your wife, sisters!
Stay Safe!