OK - time try again tackling again with MIL - call her today - say you are worried about this weekend, that DD1 has really been struggling with how much pain she's in when you have to get tangles out of her hair, that it's taking the best part of an hour to detangle and you really need MIL to help you this weekend. Say you will send the girls with their hair plaited and can she make sure she doesn't undo the plaits. Repeat several times, the girls have been explained to this is the only way to avoid the painful brushing (say 'pain' 'painful' 'hurt' several times to reinforce the message). Ask her if she can make sure she doesn't take the plaits out, that's all you need her to do.
Tell DD1 that she must not take her plaits out and that you've told Granny not to take her plaits out, but if she forgets, DD1 is allowed to remind her not to take the plaits out to stop it hurting.
When MIL picks them up, in front of DDs, say "And Granny, they both know not to take their plaits out to avoid it being tangled and hurting when it's brushed out, so girls, what must you do?" (not take our plaits out). Big smiles to MIL and "hopefully they'll be good for you, if they do take them out can you replait to avoid it being painfully matted?"
Repeat, repeat, repeat. Hopefully the nagging approach will work.
If it doesn't work, when your DDs arrive back, ask "what happened to their hair?" If your MIL genuinely doesn't care that her behaviour causes her DGDs pain, then she doesn't get unsupervised access until the girls are old enough to look after their own hair.
If your DH is going ot be there too, you have a different issue - I would make him do the de-tangling or at least sit with you while it happens and watch his DD cry in pain due to something he's caused. Don't sugar coat it "why aren't you doing something to stop your daughter being in pain? Don't you care that you are hurting your daughter?" This is horrible of him to allow his child to get hurt.