significant other or sibling

I am not sure if i understand your question but I'll ive a try. I love my bf as a lover. If in a couple, one thinks his/her partner as a sibling like more of a brother or sister or just a friend, then maybe there is less chemistry between the 2 of you. Maybe you can spice up the relationship. Try out romantic things and kiss alot lol. But if there is just no more chemistry, then maybe the couple should just split up and be freinds. But before splitting be sure of your feelings inside you.