Just like Sita accompanied Lord Rama during "vanavasa", were it not the dharma of Urmila to accompany Lakshmana?
This link gives details about Laxman's wife Urmila and it also states that she was serving Lord Ram and Laxman's parents, as the eldest Daughter-in-Law of their family after Lord Ram Laxman and Godess Sita left for the forest life (vanvasa) of 14 years.
According to Ramayana, when Lakshman was leaving for the forest with lord Rama, Urmila was ready to accompany him. But, Lakshmana hesitates and advises her to stay back to take care of her "in laws".He also states that he cannot take care of her in the forest as he will be busy serving his brother and apologizes to her. Urmila agrees and remains a devoted wife for fourteen years until her husband returned
Source Wikipedia (Urmila)