Would you consider this a red flag?

When I was 9 or 10, my stepdad came to visit us before my mom and he were married.

My little sister, Gretchen, had just learned how to tie her shoes. I thought this was a big achievement and milestone. She was 5 or 6 and has Down syndrome. I was proud of her.

Stepdad leaned down to tie Gretchen’s shoelaces. I said, “she knows how to tie her shoes.”

He stood up, slammed the door, and went for a walk. That was my first encounter with his anger.

Would a reasonable person see this as a red flag?

I was told that he was being nice to Gretchen by tying her shoelaces. Was he nice to me?