We have just had this happen also.
DH says the ewe often will think she won't have enough food to be able to produce enough milk for more than 1 baby, so she will reject the weaker one in order to let the stronger one survive.
Sometimes she will accept the little one after it has nursed and the milk has passed through...it puts the ewe's smell in the baby.
Our ewe knows this is her baby, but still was butting her away. We just hold her and let the little one nurse. She is learning that if she pays attention to when her brother is nursing, she can nurse at the same time without getting pushed away. We held the ewe as often as we could, and checked the lambs tummy before she nursed every time. She had been sneaking some milk, because she has not been drawn up at all.
If you are able to, I'd continue holding the ewe. I wouldn't get up all hours of the night, though. She will be fine until morning. Just make sure that you keep the ewe contained so she can't lead the lamb off and hide her, and never go back to get her.
Sheep milk is very rich, and it is hard to find a decent milk replacer, in my opinion. Mom's milk is always better, and I don't think it is any more inconvenient to hold the ewe than to bottle feed.
Ours were born on March 8th. This is the first time this ewe has had twins. She has had singles and has been an excellent mother in the past. We kept this ewe and the lambs in the barn for 2 weeks before letting them out with the other sheep. We only hold the ewe twice a day now, when we let them out in the pasture in the morning and when we bring them in at night. She doesn't nurse very long before she goes off to jump and play with the other lambs. I do see our little one nursing during the day out in the pasture, but only when her brother is nursing.
Last year we had the same thing happen, and I had a goat that had stillborn twins the same time, so I just rubbed one of the dead kids all over the lamb and gave her to the goat. Now I have a yearling sheep who acts like a goat...LOL. She jumps into the back of the pickup, on top of rolls of hay, and anything else she can get on top of!
I'm sure your little girl will be fine. Sounds like you are handling it just fine.
Jean