Dilly put the weaker kitt on the best teat and supervise so she gets a good feed,and all being well she will gain at least a little strength and also syringe feed her with a little egg yolk mixed with a little honey
You need to do something because the others will have sensed her weakness and she'll just keep getting pushed out-it is very much survival of the fittest
The others are eating as well so they don't need mum as much,also a little sardine mashed into a little rice pudding-served at room temp might help some