Sounds like mum has milk if the other one is doing okay.
I'd have the whole family indoors in the warm and I'd be feeding the little one at least 6 times in 24 hours. If you can get 0.5L into it over that period it should be ok. Personally I bottle feed with the non-vac soft teat bottles; I have not found it interferes with the lamb getting back onto mum, quite the reverse. But I know others have different experiences on this. And if it won't take as much as I think it needs then I tube more into it.
Ideally feed colostrum for the first 24 hours and then you can switch to milk powder if you must - but if you've got mum there and can milk her, use that. The early colostrum is very thick, like egg custard! but it should be more runny by now. You can always mix it with powdered to make it easier to get it to flow out of a bottle. And keep trying to get the lamb onto the teat - if it will suck a bottle it should suck mum.
If its sibling is getting stronger, it may need your support to get its share for a while. If you can get it to suckle, stand with it to make sure the other sibling isn't pushing it off before its had enough.
Enough is when its belly is plump and round and when you leave it, it curls up and sleeps happily. If it stands hunched, and looks collapsed in front of the hind legs, it's hungry. If you're not sure, stand it on its hind legs and look at the belly; it should be plump and round.