It's a sad situation... but you can't force them to love both grandchildren equally.
You can, however, ask them to modify their behaviour. They shouldn't be saying negative things about DD1, comparing the girls or giving DD2 extra attention when DD1 is present. But IMO it's unrealistic to expect them to treat both exactly the same. Why should they be expected to have DD1 for sleepovers if she misbehaves or is reluctant to go anyway? Why should they force her to be in photos if she doesn't like having her photo taken? Since your DDad is so unwell, it might be too much having both girls at once, they may feel unable to cope.
They clearly have a much stronger, closer bond with DD2 and this is no-one's fault, it's just how things have turned out. I doubt they set out to favour her over DD1, but they've spent a lot more time with DD2 due to DD1 being in nursery, so it's natural they feel closer to her and understand her better.
If you confront them too aggressively or insist they have both girls for sleepovers, they may back off completely and your DD2 could miss out on what sounds like a very close loving relationship.
Could you suggest they find other ways of bonding with DD1 and making her feel special, instead of expecting them to have her for sleepovers or babysit her? Are there certain activities DD1 likes doing, that they could do with her? Could they take her for a day-out without DD2 present, so she has some special time alone with them? Eg a trip to the zoo or a meal out. If DD1 doesn't like being away from home, I imagine it's very stressful trying to comfort her and calm her if she gets homesick and moody. Could they maybe see her by herself sometimes, just for a couple of hours rather than overnight, doing something she enjoys?
When I was a child, I had a very close relationship with one grandma, who favoured me over my older sister. I was quite fiery and difficult as a child, and this grandma was also fiery and passionate by nature... we understood each other! We had a wonderful bond, she'd often have me to stay by myself, and I was allowed to be expressive, make a mess, go wild, be noisy etc. My sister was far more sensible, reserved and quiet. She was a bit scared of this grandma. However, my sis was much closer to our other grandma than me, would visit other grandma without me and they shared lots of interests. Other grandma found me difficult and preferred to have my sister alone. I don't recall either of us feeling jealous or resentful.
So what I'm saying is sometimes grandparents bond more with one grandchild than the other, either because of similar personalities or they find one easier. It would have been a real shame if my sister and me had been prevented from close relationships with our grandmas just because our parents felt they didn't love us equally, or made unreasonable demands about having both of us for sleepovers.
Are there any other relatives in your family who have a close bond with DD1 and could spend more time with her so she doesn't feel left out?
It's not compulsory for grandparents to love their grandchildren at all...or have them for sleepovers. Many don't. So I think when there is a special bond, it's something to be treasured and encouraged for the child's sake.
Provided your DD1 isn't being excluded or treated meanly, I think it's worth letting the grandparents keep their special bond with DD2 and try to help them find new ways of getting to know DD1 as well, without putting pressure on them to have both girls together or for sleepovers.