That is how the novel ends. Saras marries Kusum on the insistence of Kumud. But the book was based in the 1900 where divorce, remarriage and love weren't a part of the society. The show is based on the ideals of the 21st century so this may not be the end of the show. I don't feel that Kusum would agree to the loveless marriage nor would Kumud want her sister to be out through that unless the circumstances are very extreme.
I would say that Saras and Kumud would be together by the end of the show because of all those times the writers have emphasised that they will be together. The show is trying to depict how they find true love and cannot give it up no matter how hard they try, and SLB wants to complete an incomplete love story, I really don't think he is going to end it in the same way, it's an "adaption" not a remake!