Simple suggestion.

Touch and taste and smell are emotions you'll never be able to replace. Sound and sight are easy. Does your wife have a favorite flower or perfume? Try to incorporate those things (particularly a flower or food perhaps) in the time your kids spend with your daughter (you too!) Those associations simply can't be replicated.

Also, if your wife has the strength to physically write with a pen on paper, try to see if she has it in her to write some letters directly to your daughters.