Canto 2 -
Ayodhyā-kāṇḍa
Chapter 26: Rāma Meets Sītā and Instructs Her
Text 2.26.34

विप्रियं न च कर्तव्यं भरतस्य कदाचन।
स हि राजा प्रभुश्चैव देशस्य च कुलस्य च॥

vipriyaṁ na ca kartavyaṁ bharatasya kadācana
sa hi rājā prabhuś caiva deśasya ca kulasya ca

vipriyam = displeasingly; na ca kartavyam = you should never act; bharatasya = to Bharata; kadācana = ever; saḥ = He; hi = because; rājā = is [now] the king; prabhuḥ = master; ca eva = and; deśasya ca = of the country; kulasya = the family; ca = and.

You should never ever act displeasingly to Bharata because He is [now] the king and master of the country and the family.

Even though one might be a king, he might not be a master, that is, a controller. To ward off that possibility, Rāma mentions that Bharata is the master. And it might be possible for one to be the master of a country, but not the master of the family. To ward off that possibility, He mentions that Bharata is the master of the country and the family.