Sarita and Mark want to have a child, but Sarita is infertile so her sister Reena offers to be a surrogate and carry Mark's child. Sarita initially agrees but eventually decides she is not ready for motherhood. Unfortunately, Reena is already pregnant and she decides to go ahead with the pregnancy. This is not a popular move - Sarita stops talking to her sister and her husband, while Reena's girlfriend Lisa moves out. The only people still talking to each other are Reena and Mark who become close for the first time.
The ending is rather vague, but determinedly happy. One is left with the impression that Reena and Lisa (who have reconciled) raise the baby as their own, while Sarita and Mark will be closely involved. It is never explained whether they decide to go through with Mark being the official father, or whether they are raising the child as entirely theirs.
Thanks Mary C!