As she has now bonded with her mother, and you are resigned to keeping them in anyway, I would make her a pen to the side of the mother, so they can talk and mum can sniff and lick, but the lamb can’t get to the mother’s udder. Keep them all together overnight for comfort and security, pen the lamb in her own pen first thing. Offer her her first bottle two to three hours later. Keep her separate like this during the daytime, put her back with mum after her last feed when you go to bed. Hopefully she will begin to take better feeds from you during the day, so you can maybe get her to take fewer, larger feeds. (At 17-18 days old she will be fine with her mum overnight, no need to do a middle of the night feed
.). Once she starts to know she needs the bottle, hopefully she will begin to call for it and run to you, then you can try her loose with the family again and see if she will run to you as your other top up lamb does.
An alternative, if you are sure the mother has sufficient milk, is to support the lamb to feed from the mother. Instead of bringing a bottle 6 times a day, go in four times a day, steady the mum and let the lamb feed at will. If there’s a sibling who is taking all the milk, keep both lambs in a pen to the side (as above) during the day, bring both in to the mother’s pen to feed four times during the day and leave them all together overnight, as above. After a week maybe, the mother may let the lamb feed unsupported.