My 5-year-old girl is not fond of my brother's 13-year-old daughter. She was born in front of me and I was with her during her growing up years, and my best niece. How do I help bond them?

True...your daughter might be harbouring some jealousy towards her cousin. Afterall you are her mother and if you are overly affectionate towards your niece in front your daughter she will not like it. No matter how close your niece is to you. Try not to force a bond between the two...they will develop their own bond in time.