Best Friend

I been friends with my best friend for 16+ years.... And I have always liked and loved her brother for some reason... When we were younger we never dated or did anything just a friendship... From December 2015 until now we been spending more time together(her brother) and on February 22 he asked me to be his girlfriend... Well m happy because I finally fee complete and oil have always loved him everyone tells me he my true love... Well now my best friend tells me that she feels like she losing her best friend(ME) please give me advice I don't want to make her feel that way... I love her she more like a sister to me.... But I feel like in a battle with my best friend and my boyfriend(her brother)... What can I do to not make her feel that way ??? I don't want to lose her friendship but I don't want to lose him either...