how do i tell my girlfriend that i am in love with her sister

how do i tell my girlfriend that i am in love with her sister. me and her sister love each other. they are sisters for Gad sake!!!. and she told me that her most hurting experience was when her very own sister took her previous boyfriend. now she has taken me also. she is just so caring, so kind, so concerned, her character, she acts like a wife. i totally see why her sister's boyfriends switch boats. now its me. how do i tell her. she may think shes just cursed to loose her men to other women, her emotions as a woman or self esteem could be hurt. what do i do?