At this point, it's more about getting the fluids into her.
Jello is considered a liquid (You know that from the I&O Sheets!)
Also, if it's a teething problem, you can do double duty by giving her frozen juice pops!
I bet you can even freeze Pediasure® into popsickles.
Is she eating solids?
My daughter had a problem for a while. Enlarged "kissing" tonsils. Stuff would get "stuck" on them when she ate, causing her to gag. It took a while and a lot of experimenting to see what she could tolerate.
As for your Mother-in-law...(where do you think all those jokes come from?!) INVITE her to come help you feed her!
She is no doubt feeling insecure about the whole thing.
Let her SEE how you ARE ahndling things. It may help.
I know...helping HER to feel more secure SEEMS like it's eating away at your credibility...but in the long run, you'll be showing her that you "respect her opinions". Then, when she goes home, you can resume your normal life. (grin)
It's GOT to be better than mid-day panic calls!