Help with my feelings?

Me and my stepbrother, tim,( idk what else to refer him as because we live together but our parents aren't married) have been living together for two years. We're best friends, we're always laughing and are able to read eachothers thoughts cuz we think very much alike. A lot of people think we're a couple even his two cousins asked if I was his gf when I first met them. My friends all think we're together in some sexual taboo thing lol. Tim's always all over me which I don't mind to be honest, I like it. But we both have lovers and I notice that whenever I have ian (boyfriend) over tim becomes quite and tense and gives ian glares a lot. I've been trying cover up my feeling for tim for two years hoping that it'll go away but it hasn't. I've made myself go into a relationship with ian though I love him, I can't ever find something we both like. Its like whatmy friend made me realise, I can see myself living with ian for a short time, but I see me marrying, living, and having kids with tim the most, and I feel happy and safe when I think of it. It confuses me because this must be wrong to feel like this for him. And I try so hard to make my relationship with ian work, I cook, buy and try to do things for him, and he does a lot for me, he's a great boyfriend. Its just that recently tims been making weird actions and saying things in halfs. Like one time we were talking and I said "I'm gonna move and rinana (his gf) will live here with you"and I didn't finish cuz he said "no you'll always be here" and then he changed the subject. Then today in the car I said "my house is gonna have a cherry blossom tree" and he said "no it won't be in my lawn women!" He knew he said something he shoudnt have so it was awkwardly silent until my mum came back. These things make me so confused. I just need help there's something messed up with me!