I know about Older Sister Syndrome. But have you ever heard of this happening? [View all]

As adults, older sister turns siblings against mother so as to become the mother figure of the extended family, and shut mom out?

That has actually happened to me. I just figured it out.
She abducted my special needs adult daughter almost two years ago.
She has turned my sons against me, making up a weird narrative that puts her at the top of the family.
She's now talking about commiting my special needs daughter to a residential facility for people w disabilities.
She has NOT tried to obtain legal guardianship, by the way, of special needs dtr.

This seems to be an exacerbated Older Sister Syndrome type of thing where she acts to draw siblings to herself and shut out the mother. She has always been super bossy -- her siblings called her "Hitler" when they were young.