Places to sit/rest in museums and palaces

It is not but even in the US benches can be infrequent. Depending on the situation it can be a long distance between benches especially in palaces or where you are going from one small room to another. In larger art galleries and museums, benches are fairly common. Even when there are benches they are often full of other people.

I was going to say, it's just like in the US - meaning you can never count on seating in a museum. Some rooms will have them, some will not. And when you need to sit, even if there are seats in that room, they may be full.

If your sister is going to need to sit down, I suggest one of those canes that has a folding seat, like one of these: https://www.amazon.com/s?k=cane+seat&ref=nb_sb_noss_1

I've seen those canes (and walkers) in use, and they work great, but they are frowned upon in some places with expensive museum-grade parquet floor (Hermitage, some other places - basically, any place that makes you wear those less-than-fancy shoecovers).
As an alternative - you could get a (free) wheelchair, where available, but then you'd probably want to be wheelchair-bound for the duration of your museum tour.

The Hermitage and Russia museums both have wheelchairs you can borrow. They are hugh museums and as said above, the few benches that are there are usually full.