we never had a new one develop in our internal system. short version all my alters were created due to extreme trauma. each of my alters had their own sense of agency ( jobs, purposes, reasons for being created how much control , co consciousness and so on that they had) as a result of this kind of internal system there was an alter that made sure that needs were met as a way to prevent being harmed by crying, or otherwise making others outside of my body know they existed or that the body born or those resided with in needed care taking or other needs.

example if Rainy was crying because she was scared, then eagle would take control and make sure there was no danger near by and be able to move swiftly out of harms way,

another example if Red was close to throwing a temper tantrum because she needed a drink of water Sunny who knew how to do that took control, got the cup down and put water in the cup, then red took control back and drank the water.

All of my alters were like this with their own sense of agency, the system was set up in what ever ways was needed in order to survive.

my suggestion is since you know this alter is asking another inside the system to be their mother you have some sort of co consciousness with your alters. maybe you can also ask inside to see which of your alters sense of agency is to make sure every ones needs are met. maybe you can ask this / those alters if they can be more attentive to the systems needs because you are not able to do that for your self at this time.

or if you can do things like self nurturing self parenting maybe doing whats needed for you will filter down to this alter who feels they need a mother figure.

example sometimes if I could feel Rainy needed to curl up with her blanket I (body born) would find her blanket and sit on the sofa wrapped in the blanket, this self cuddling time filtered through to rainy.