Warning - graphic.
I'm so sorry to hear of your daughter's loss. However, it is to the benefit of the other 2 babies to leave the demised one where she is and try to sustain the pregnancy for as much longer as they can. They just can't take one out and leave the others to gestate longer. (I can't say it's never happened, but that's just not how it works - removing one is going to prompt the birth of the others as well.) I really don't think the damage to the other baby is caused by the death of the first, although they might have a related cause. The demised baby isn't going to hurt anything, not like being signifigantly premature would, even if the idea makes you a bit squeamish. She'll be born later with her siblings. My sister had a baby die during a multiple pregnancy. One died at 26 weeks and was born with her sister at 35. She was not decomposed in the way that you might be imagining, but rather petrified in a way. We had the opportunity to see and hold her after birth, and while she looked a bit mummified and alien, it wasn't gruesome or frightening. She was poorly positioned transverse across the cervix at birth, which necessitated a c-section, but did not pose other complication to the birth. A vaginal delivery had originally been planned. The risks involved, as I recall, possible clotting to the mother in reaction to the dead fetus which was treated with bloodthinning injections, as well as preterm labor which in her case was held off with a cerclage and a terbultalin (sp?) pump (for regular dosing of medication to hold off contractions.) She remained at home by choice on bedrest during this time, as she felt little would be gained by being in hospital.
Sending my warmest thoughts in hoping for the best outcome for your other two grandchildren, and hoping you can find comfort for your loss.