I haven't really watched these episodes in order, so I don't know if there was an episode that introduced her or not. Did Frey even want a maid? Inga didn't seem like somone he wanted.

Inga was introduced after Thelma went into the army after her "non-wedding" to Reuben. I think Ernie paid Inga's salary with the gift money Thelma and Reuben got at the bridal shower. I have those episodes on tape somewhere - I should dig them out so I'm more clear.

I think Inga had a thing for Mr Frey