My MIL and her sisters asked their mom when their baby sister was born if they could call her Cookie. The mom said yes just figuring it was child funniness. The thing is 50+ years later the baby's name is still Cookie.
Our old was 2 years 5 months when her sister was born and she couldn't say Amelia so she called her Mia. #2 dd doesnt like to be called Mia by anybody but her immediate family so we must be sure to introduce her as Amelia. We even had to tell the grandparents address everything as Amelia.
#3 dd for a very short time period couldnt say Lydia so she called her LouLou. Well dh and I like that nick name for some reason and have taken to calling her that at times.