Based on the promos and what the maid said in the first episode it seems like Satrupa doesn't even know that she had twins. I think she just thought she had one. Or maybe she does know.
Whatever it is, Anami is a girl and it seems that if she had lived then she might have been the older child and thus the waaris, which obviously can't be because she's a girl. So if she had stayed in the house, she would have been killed or shunned or something along those lines so that the boy could be the waaris. Now either Satrupa gave Anami away or the maid decided herself to give Anami away in a bid to save her life.