My mother in law has a septic tank and her toilet backs up into the shower? What can cause this? She was supposed to have had the septic tank replaced lasty spring.

A blockage anywhere in the sewer line between the toilet and the septic tank.
Why would she replace a septic tank unless someone drove a truck into it? Replace distribution lines maybe but why tank?

Wondergirl
and I'm just a dumb girl.

A girl... I'll buy that.

Dumb... no way under the stars.

It sounds like your mainline is blocked. So when toilet is flushed it comes out of lowest drain.

And by the way, septic might be full.