When this brother is mad at his sister, he asks Reddit:
Sis is 28 I'm 26M. My sis N has always had a strained relationship with our parents especially my mom. I am clearly not privy to the reasons because things are fine with me and my parents.
When N went to college she met her creative writing professor as a freshman and they got close immediately. They would do a lot together and worked closely on a few different writing projects.
N never specifically said this, but it was obvious to anyone who saw them interact that they had a substitute mother/daughter type relationship. Which hurt my mom a lot to see.
I always thought she'd grow out of it or that the prof would move on but ten years later they were still very close.
About a month ago the prof died unexpectedly and it devastated N. She was really dperessed over the holidays which of course was all in front of my mom and was a difficult reminder that N loved the prof as a mother way more than she ever loved my mom as a mother.