Can a person that has epilepsy and a bipolar disorder have a mate, get married and have children like others?

The reason why I ask is because I have a cousin whose daughter has epilepsy and is bipolar and she told me that her daughter's doctor advised her that it would be best for her daughter if she never was allowed to be involved in a relationship, never get married and never have children. He claims she has the mental actions of an 11 year old and she is 20 years old. To look at her a person can't tell anything is wrong with her and she can do things for herself and cook and she looks and acts to me like a typical 20 year old would. I just think it is cheating her from having any chance at a life of her own by her mother keeping her under her wing 24/7 and keeping control of her and the daughter always being under her thumb. She's missing out on the chance of falling in love and having children, grandchildren for her mother to also love and enjoy watching them grow up and be around. Please let me know what ya'll think about this situation. :0)