What happens when one twin dies in utero?

Ive noticed that my niece is slightly more advanced than most babies her age. since she was only a few months old shes been at least 3 months advanced than her age... we have been trying to research if the fact that her brother died when my sister was only 4 months pregnant. She hates sleeping alone and if you leave her for more than a few minutes she will wake up. And she has 2 very different personalities, one minute shes a ham and loves acting like a little clown which neither of her parents really are the next shes as serious as both of them combined. We are trying to see if she is possessing both her and her brothers personalities to make up for the loss.

Answer #2 I am a twinless twin, my twin died also in the womb at an early stage. But as a baby, I was considered to be very smart. I grew up figuring out things adults even couldn't understand. I'm also multi-talented and have a very unique personality. Different sides. And I feel something a lot. I have no idea what it is, but something tells me others don't feel this. It's like...a strong feeling, that makes you stronger. If I had to guess, I would have said my twin's traits or energy transfered over to me.