Ok but my parents are so worried about my sister marriage because her age is (removed). They are afraid that if they reject this proposal then in future what if they don't get another good proposal and have to wait a long. We are living in a conservative culture where everyone questioning of girls marriage. The situation becomes more difficult when girl is wholly dependent on her parents. And when parents gets old they only want to see their girl to have married to responsible person. My parents only concern about what if GOD forbid something happend to them then who will take care of their daughter who is no career oriented and totally dependent on her parents. How she live alone her life while she never stepped out of her home alone in her entire life.

Is it necessary that all girls get married? This is in the hands of Allah SWT. You can not force them to marry someone they do not want. Let them depend on Allah SWT. Allah SWT knows better than what we do. Do your parents believe that they raised her? Let us believe in Allah SWT, and just pray to him to do for her what is the best for her.
No body can guarantee that the marriage is the real protection. Many times we think that this is the right man for our daughter, or this the right girl for our son, and then we realize that we put them in the hell with this marriage. I can give you many examples. Hence, let us just pray for her, and leave it to Allah. On the other hand, if her parents passed away, you as brother can take care of her, and this is the true meaning of brotherhood.