I am so very sorry that the baby died
{{hugs}}
Babies are weaned between 4 and 5 weeks old and baby girls need to stay with their mama until they are at least 6 weeks old.
Rats need to live with at least one other rat ….. and babies need another baby to play with
so you must get her another rattie girl to live with - preferably one of her sisters
What are you feeding the baby?
You may want to supplement a good quality block (oxbow or Harlan Teklad 2014) and daily vegs (there is a forbidden foods list) with organic soy infant formula thickened with organic baby cereal - the formula should be at your local grocery store in the infant section, if not it can be ordered online from Walmart
babies often can not get water out of water bottles even tho they look like they are drinking because their tongues are not strong enough to move the ball bearing with all the water pressure behind it. So in addition to the water bottle, use a water dish that can not be upset ….. rats need at least 2 sources of water in their cage and water dishes need to be cleaned a coupe of times a day
You should be aware that rats are not inexpensive pets and frequently require vet care from a good vet with the knowledge and experience to treat rats. I hope you have parental support for your pets because vet bills and medications can be expensive. Rats get sick and die fast so when you notice your rat is ill, you are well into a medical emergency .
I hope your remaining girl is ok, and that you can quickly get her another baby to live with because she can not be kept alone.
You should also consider getting her spayed when she is a bit older if you have access to a good vet with the knowledge and experience to safely spay rats
https://www.ratshackforum.com/threads/why-spay-or-neuter-rats.35402/
good luck with your girl and with your mom