We have a tiny twin Suffolk lamb we pulled soon after her birth. She was half the size of her brother at birth and her eyes were stuck shut so I think she's not 100% right to begin with.
She was hypothermic as it was really cold out and, she didn't stand right away. Her mother didn't clean her, although didn't outright reject her either, but chose to focus on her more normal brother. We warmed her and fed her colostrum from her mom through a tube the first day. We gradually got her to take a bottle and she started to act almost normal, but not vigorous.
I think she may have picked up some joint ill from our dog licking her umbilical cord so I gave her some antibiotic for her swollen knees, but now she's going downhill again and I'm back to tube feeding and she doesn't walk well anymore, but we can still get her to stand for a while and take some wobbly steps.
She's about 10 days and has barely gained any weight, although her digestive system seems to be processing food just fine, based on what comes out. I think while my kids were bottle feeding the first week, she wasn't getting enough and it weakened her over time.
My son thinks we should take her to the vet, but I don't have a lot of hope left and am just trying to keep her fed and comfortable.
Heather, it sounds as if you have done a great job with the lamb to date. And the answer to your question is to never give up on a lamb, at any stage. But then I think you knew the answer to that question, any way. ;)
With regards to the lamb having her eyes closed when born, and being so much smaller than her brother, it sounds as if your ewe has a double uterus which caused her to have 2 pregnancies but at different times.
The uterine contractions of the full term lamb caused the other lamb to be born at the same time and clearly she wasn't ready to be born. Hence all her problems. This is of course just a guess, your vet would be able to confirm this if he examines your ewe. If so, I would advise against breeding from her again.
By the sounds of things the lamb doesn't have scours which is one good thing! The only other thing I would do is to add a teaspoon of cod liver oil to her milk. This is an excellent pick-me-up for your lamb and I would do this for a couple of days until she is able to stand properly.
I would also make sure that she gets all her feeds, as she is too small not to get what she needs. Weigh her daily, and keep a record.
Not being there, it is hard to tell if she needs a vet at this stage, only you can be the judge of that.