I think Dorothy would be a lovely middle name! I suppose it doesn't flow perfectly after Snow, though... 'Dorothy Snow' sounds lovely but if you don't want it as a first name that doesn't work, ha ha. I agree that something which sounds like Jewel, such as Jules or Juliet, would work really well!
You could also look for variations on the names. Some of Dorothy are: Doa, Dolly, Doe, Dori, Dorrit (Dori Snow sounds cute to me!). Or look for names which have the same meaning: gift of God. There's a lot of names which mean that, so you could take your pick, and it would be a great way to honour her memory while also choosing a name you like a lot!