The Tale of Two Sisters

Since i was little my mother always taught my sister and i, to watch out for each other. She knew that because we were sisters we would always have each other. So unlike most siblings we became best friends. Since my sisters graduation three years ago she has moved to Boston and is attending Wellesley College, an all girls school. My sister always felt the need to prove herself to my parents, as well has prove her independence. The move was good thing for her. Yet for me, it was hard. For 18 years i wok up everyday and went to bed knowing my best friend would be there in the room next to me tomorrow. Since her move i have seen her grow, learn, and become one of the most amazing people I've had the pleasure to meet.
Her spring break is after UNK's so she decided to come home, see her parents and come up to UNK for the weekend. I have had Morgan here before but each time I'm equally as excited. Since are worlds our worlds are so far apart, whenever we get together we cant wait to share are new stories with one another. Shes met my friends, seen what my friends are like, and has opened up to the idea of Greek life. Yet my sister and don't agree on one things. Boys
Since her move to Boston she has had an on and off again romance with a boy from MIT. His name is Taylor. Taylor, in my opinion long stream of bad words id rather not repeat. Like any relationship he has played stupid mind games with her and led her on. They stopped talking for a year.
Then about a month ago Morgan calls me to tell me she has a boyfriend. "Its um...Taylor." I almost lost it to say the least. For me this meant that my sister was once again with the man that i hated. Yet she assured me that things would be different this time around and that. So like my mother taught me I respected my sisters wishes and agreed not to talk shit every time she spoke his name. Despite my love for her and respect for choices, i cant help but disagree with her choices, kinda like she would for mine. So how do you tell your sister that shes dating a complete idiot who will only break her heart and leave her a mess, without sounding like our mother?