Is it true that sprinkling an unmarried girl with water used to wash a deceased person, even if it's just a light sprinkle, prevents her from getting married for life, and does Ruqyah Shariah not benefit in this case?

This statement is false, for marriage is a provision (rizq) granted by Allah to whomever He decrees it for. What Allah has decreed cannot be prevented by sprinkling water or any other action, except by Allah's permission and decree. Allah Almighty said: "Whatever mercy Allah grants to people, none can withhold it; and whatever He withholds, none can release it thereafter. And He is the All-Mighty, the All-Wise." (Fatir: 2)