Honestly, I wouldn't worry about being "too closely bonded" with your sister.

Developmentally, the only time I would begin to worry about the connection you have with your sister is if you become100% co-dependent on each other. Bonding and forming healthy relationships in early childhood is an important part of ones overall development. Between 0-3 years old is when a child grows and develops at the fastest rate. The brain grows from being 25% formed to 80% formed and has formed twice as many synapses (connections between neurons/nerve cells) by age 3 than it will have in adulthood. Early childhood experiences are what form the synapses; the synapses used will strengthen and, throughout life, those that are not used will eventually disappear in a process called pruning. During the first year of life trust, the foundation of all relationships, is developed. "The more loving and responsive the caregiver is, the greater the foundation for later social interaction."

As far as healthy, happy familial relationships are concerned, as long as you...

-- maintain and reinforce that you are her sister (i.e. Don't let her call you mom or something similar)

-- respect her mom's decisions as a parent

-- ensure that you create and maintain an open line of communication between yourself and her parents to avoid stepping on toes and/or over stepping boundaries

... I see no reason for concern.

Your sister, her mom, and (I'm assuming; correct me if I'm wrong) your dad are incredibly lucky to have someone as loving, caring, and thoughtful as you in their lives!

Peaceful Parenting Support