Why are sisters more brutally blunt with you than everyone else is?

My sister isn’t afraid to tell me the hard truth. She said she knows me a lot better than anyone else and she knows my weaknesses she doesn’t even think that I’m ready for a serious commitment in a relationship. I keep telling her that she is wrong but she keeps saying that she seen goes to get vantage of her and she doesn’t even stand up for her self and she says I’m not willing to keep a relationship up. She really knows how to shut me up with the truth. Siblings are “meaner” than there.