Why do they call Renesmee Cullen Nessie?

She combined Renee, which is her mom's name and Esme, the motherly figure of the Cullen family, so therefore Edward's mom and her mom combined would make Renesmee.

And it's not in Twilight that she names her, it's in Breaking Dawn.

If the baby was a boy it be called Ej which is a combination of Edward (The baby's dad) and Jacob (its uncle figure)