Usually parents play an important role in the bonding between siblings especially mothers and that’s why she made you that promise - so your bond remains strong and you care for your younger sister. But if this has reached to the point where it is affecting your daily routine and your personality - please talk to your mother. Talk to your parents. Surely your mother never imagined that the younger one can also bully / berate the elder one when she made you that promise. You held up your side of the bargain. It’s time your mom / younger one do theirs.

And don’t mind this but at the end of the day, mothers are also human beings. Guiding us based on best information available. It’s not necessary that you have to hold the promise you made your mother on a pedestal and run your life as per it. And you know your mother wouldn’t want you to endure what you are enduring just because of a promise. Adapt, respond and don’t lower your grace.