How should I end my sister's relationship?

Your mother and father are not willing to budge your sister's relationship and they are the parents here. So what can you do? Talk to your little sister. You need to state all reasons why she cannot be with this 25 year old man. Be the older stronger sister, and let her know that what she is doing is wrong. Don't just think about what you're going to say to her overnight, take a whole day to think it over in your head and make sure you cover everything. Start and finish what you say by letting her know that you love her so much and you care about her. Always be there for your sister no matter how much you feel like smacking her face. Once her and her guy gets into an argument or disagreement, she'll know who to turn to when it gets tough and she will turn to you. Always make the time to call her up just to see what she's doing and just to make her know that you care about her. Best of Luck!