transitioning from bottle to sippy cup for milk

Steph

our twins are 13 months old and for the most part get their milk intake need from the bottle and not sippy cup. I warm the milk and it's lukewarm. I've tried various brands. I have also went down a nipple speed and I'm thinking I might need to do that again to help entice them to get it out of the sippy cup. any suggestions would be great. our pediatrician would like them off of bottles by 15 months