I'd give her vitamins if you have some. When you say she's eating, is she eating hay as well as hard food? Does she have and is she drinking plenty of water? I'd expect a ewe indoors with twins to drink two buckets of water a day, at least, and she'll need more if she's scouring.
As you're new to this, then I would chat to the vet, yes. S/he may say to bring some faeces in for analysis, or may want to see the ewe.
Fingers crossed she's okay - and congratulations on your first lambs!