In my story the mother explains to her son her secret past. Who she is, were she came from, how she met his father and what happened before and after she met his father. When I wrote it down her explanation comes to about one page long. Not certain if a one page explanation is a little to long.
Two words of explanation is too much in a story. A substory inside a story... No limits. Don't make it an explanation but a substory.
Why waterboard your readers? Give them widely separated sips, and they'll eagerly await the next one, not curse you for nearly drowning them.
Depends on the length of the work and the skill it is told in. The key is not to make it seem like an info dump in any event.
I’ve read plenty of books with one pages explanations. I’m reading “Aru Shaw and the End of Time” right now, and her mentor infodumps hardcore, but it’s cute and interesting so it’s fine. If the people who count to you like it, it’s fine.
"I'm not your older sister I am your mother. And Mom and Dad are your grandparents". That could take a page or two.
