Aww that's so sad 
First I would like to say it was really sweet of you to take those kittens in the house even tho your roomate is allergic to cats 
, but since the kittens need their mother up to 6 weeks cause of their mother's milk and you were unable to find their mother and one of the kitten is become weaker or dying is may be cause of the whether or the kitten wasn't feed by the mother or it could be alot of other reasons why that kitten wouldn't move....Another thing I would like to add is never give a cat/kitten milk, because it will make them sick but kitten do drink forumla which you can get it at the pet store...And to answer your question on about the mother cat, she knows you have her babies and may want to be near them if you did put food down for the mother and she did not eat it then you know why....
Also, you can always call the animal shelter and they will be sure those kittens are well taking care of inculding the mother cat too and the other spray cats out there also...... 
