My 6month old daughter has a dimple under each eye. she is a twin and was born at 36w. is this normal or should i worry?

Dimples under eyes: Not denoting something special, you probably are comparing her to her twin sibling, if she is otherwise fine, don't worry, that's her looks. Born prematurely doesn't have a bearing, just ask her pediatrician to have a look in the next health maintenance visit, wish you all wellness

Likely normal: Your 6 month old daughter should have been seen by the pediatrician at least 5 times (birth, 2 wk, 2mo, 4mo, 6mo) for well checks - that is a pretty good indicator to me that what you are noticing on the face is likely normal; however, impossible to say with certainty without seeing it. Ask her pediatrician who can examine her.