Yes it seems they are half sisters. Vinay draws a picture of a woman and it looked a lot like Mohi's Mother. He was very sad and had tears in his eyes, which implies that he loved this woman and maybe was forced to leave her. All this would have happened when Vinay left Rekha and Anusha when she was young and went to Bhuvana to work. (I think that is what happened). Mohi's Mother does not trust people from the city. She thinks they only betray. This would be a direct result of her experience with Dr Vinay!
Now it seems that he left a daughter behind too, which he is not aware of. Mohi's is interested in being a doctor just like her Father and sister. I agree with a comment that they should think of something new other than two sisters being paired with the same guy. But so far I find the story interesting and the concept a bit different with the girl being a tribal and all. Hope they keep the story fresh!