So I've played through Brash/Raze (with kid raze and fake raze)endings and I tried to see if I could get an ending with Sinallion but doesn't seem like that's possible? I mean the game seems to have 1 straight plot where doesn't matter which of your previous love interest you choose?. I'm just curious to know from other players! thanks in advanced.

I haven't tried for Sinallion yet, but I have noticed that the character interactions are different with whoever you previously romanced. If you're going the Sinallion route, there is an option to say your character didn't romance anyone in the first game, and I'd guess that would help.