As harmony says.
We have native breeds (currently Hebrideans but we have also kept Soay and Shetlands) and we feed them for the last 6 weeks of pregnancy - but not too much. The actual amount depends on your ground, grass, climate, current weather etc, and you soon learn to judge it. As your ewe is thin, she could have done with feed earlier. If she is NSTail then she will not carry much in reserve. I have not found native breeds to overeat, or to have overweight lambs.
I think this is a situation where the immediate actions are, as others have said, Calciject, twin lamb drench and antibiotics, even before the vet arrives. These are things you need in your lambing box. I would then be offering a glucose and salt rehydration type mix, by syringe (without the needle) every hour, plus offering digestive biscuits or similar. You can also make a porridge of ewe pencils or similar, made liquid enough to give via a catheter type syringe, which is a good nutritional support. Once a ewe has been down, she will need nursing care if she is to survive. She also needs to be stood regularly to dung and pass urine, and to burp, supporting her with bales if necessary.
I would suggest you start your other ewes on a coarse mix now, and increase the amount slowly over a couple of weeks, to be on the safe side.
I can't remember
where I heard or read this, but research shows that in-lamb ewes should not go longer than 4 hours with no feed, and all sheep should not go longer than, I think 24 or 28 hours without feed.
I hope the rest of your flock are all OK, and that the sick one pulls through.
Cross posted with MF