Step-by-step Solution:

\[ \textbf{Given:}\ \text{Sarita is introduced as ``the mother of Mohan's sister's brother.''} \] \[ \text{Let Mohan be } M,\ \text{his sister be } S. \] \[ \text{The phrase ``Mohan's sister's brother'' can refer to any brother of }S. \] \[ \text{One such brother is Mohan himself (since Mohan is a brother of his sister).} \] \[ \therefore\ \text{``Mohan's sister's brother'' can be Mohan }(M). \] \[ \text{Hence Sarita = mother of }M \Rightarrow \text{Sarita is Mohan's mother.} \] \[ \boxed{\text{Sarita is Mohan's mother.}} \]