Someone gave your Mother in law bad info. It needed to learn what it feels like to be latched from day one.
To get you on the right track: Never leave a top down for long periods of time. Put it down ,enjoy it, when your done, put it back up. Never store it down,"not even overnight"
The vinyl will get a memory faster than you know. As it ages it shrinks, gets stiffer and harder to work with.
Now to your issue: Find yourself a couple of helpers. Have someone stand outside and force down on each corner while you or someone latch that hook. Do both sides this way. It'll go!
Then leave it up and latched for awhile. Let it sit in the sun as much as possible. It will need heat to soften the vinyl. Heat lamps or large Halogen lights left on over the car can help.
Once you get it back up and latched be patient, it should get easier in time.
Good Luck