Can i marry my sister husband since she cant able to sex

Hi my name is saranya. I am from tamilnadu. In our family there are 5 sisters and two younger brother. All sisters are elder to me and are married. My age is 26 years. My brothers are now only studying 10 and 8th std. my third sister got married 5 years ago and she is not having child. Her mother in law is scolding her now and then for not having child. But her husband is a good man and also handsome. We are poor family and due to money problem only my marriage is delayed. I have no father and my mother goes for work. I also working in a shop. I have studied BSc. one day my sister came to our home. She started crying. Then we asked for the reason. She then told that she worried a lot for not having child and consulted many doctors. two years before she had a small pain near and went to doctor. She had lot of checkups. Finally doctor told her that she is having a heart problem and they told she should not have sex. She told to her husband and he said at present only the problem so it may cure in some years and he made her some relief. Then she told that his husband suffers a lot by not having sex. He is inneed of sex. She told he is not sleeping well and affects his health also. She was very much pity of him. Then she told she is thinking of it for the past 6 months and she came to a decision with two choices. 1 st choice to marry me to her husband. As she is my sister there will be no problem in living same house. And also our family have no money for my marriage. So its benefit me also like that she told. 2nd choice is to divorce him and ask him to marry another woman. Atlast she told me she not going to compell me and its my wish to take decision. My other sisters and my mother were consult each other about this matter and they told, your better choice will be marrying him. The reasons they told he is good man and give a good life. Then there will be no problem of dowry as well as money for marriage. Another reason they told if my sister got divorce and she will come to our house only and it will increase our burden. Then lastly they told you not going to share her husband. You only going to have sex with him. So there will no problem. I am confused. So please advice about this matter.my mother had become older and she is now at age nearing 60. she cant able to work after 2 or 3 years. so if i not going to accept my sister wish means, her decision will be divorce her husband and she will come to our house. it will be additional problem to us. i have a plan for good studies for my brothers. it is my duty because my parents made me study as i am their youngest daughter and my sisters also helped me a lot. they went to work in beedi company and not even crossed 8 th std. i can get bridegroom without dowry but he will not allow me to give my salary to my home. so its my major problem my brother studies will be spoiled. if i have to wait till my brothers studies my age will be 35, then no one will come to marry me. i also be inneed of marriage at that time. my sister is saying that if i marry her husband her husband will allow me for work and to give my salary to my home. she is also saying if you not like to be a legal wife, then she may divorce him and will not be in the same house, i may marry him legally. so that i only be there with him and my sister in my house. she is saying anyhow she is going to divorce for the sake of the husband and some other girl going to marry him, why not me since he is a nice person.lastly my sisters and my mother are saying that, if by god's grace my sister health become well after 5 or 6 years means, she may also share the husband. if another woman marry her husband means she have to live like a widow lifelong. please reply for this. what i have to decide.