Very touching piece - I'm sorry to hear about you losing your grandmother. The piece definitely feels in-the-moment, a proper impromptu. It was some time since I last lost a relative now, but your comments on the matter strike home to say the least.
It's hard to critique something like this without feeling like an arse.. but overall it's more than great! I almost wish for more reverb on the piano, but due to a slight hearing loss in the typical reverb response frequencies I'm not the best to judge this. I also would've kinda seen more of the cello before the orchestra entered, to really prepare for the orchestral que!
By the way, I love your progressions. You manage to pull off some great chromatic lines in the bass lines and some great use minor subdominants without sounding cheesy.