M.L.
Hello fellow Michigan mama! :)
I agree with the others- send out the invites and see what you get back. If your mom hosted it in the early afternoon, you could just serve cake and light snacks which would keep the cost down on food ( I know how quickly those things add up! ) . You'll feel less stress about offending any one and , as you said, it's REALLY hard to know where to draw the line with out offending any one. I think it's great that you are so supported and I think inviting your in-laws is a great way to show them that you still love them and want them to be involved.
Good luck!
M.