How can one deal with two sisters who are prostitutes, and is the questioner sinful for not informing her parents of this before their death?

You should make abundant supplications for the two sisters, asking for their guidance, while advising them with wisdom and good counsel, and instilling in them fear of Allah. If the advice does not work, then shun them if you hope that shunning them will bring about a benefit. And threaten to seek help from those who can deter them, such as brothers or others. As for informing the parents or others, it is not obligatory, especially if they are unable to prevent the evil.