We know, Sri Rama was Avatar of Lord Vishnu, Lakshama was one quarter portion of Vishnu [a] or avatar of Adisesha. Shatrugna and Bharatha were incarnations of Sudarshana Chakra and Panchajanya Shankha respectively.
These three brothers, Lakshamana, Shatrugna and Bharatha married Urmila, Shrutakeerti and Mandavi respectively. Rama married Sita who was incarnation of Goddess lakshmi. As portions of Vishnu were present in His brothers, portions of Goddess Lakshmi must be present in Sita's sisters.
So, how can be three sisters of Sita reconciled with concept of Goddess Lakshmi?
[a] Chapter 106 of Uttarakanda