Chair wedges to keep senior from tipping over

My Mom has advanced Alzheimer's and has trouble staying upright in chairs and on the sofa.

She slowly tips over sideways, and just stays that way!

Pillows are too soft and unstable, and rolled up comforters aren't working either.

I'm trying to find an appropriate wedge or cushion of sorts to prop her up straight on both sides, but can't find anything like that; just bed wedges and low bolsters that don't work with this particular problem as the sides need to be higher.

Has anyone heard of something like this?

Any suggestions would be most welcome.

Thank you,

Janet