HELP?!?! THIS IS BEYOND SERIOUS AND OUT OF CONTROL?

Ok, im using a friends account, she's female and im not but anyways so i have a really huge problem that's been stressing me out since last july. So me and my brother are extremely close due to the fact that we had an abusive father but now my brother, Shane, has a girlfriend that i really like. When i say like i don't mean like her like she's beautiful and id like to know her i mean like her like i wanna be with her, id marry this girl she is insanely beautiful and attractive. What also makes this situation hard is that i knew her before Shane did and i liked her from back then. Also she loves to snuggle (lol) my brother works crazy hours and is usually not home until around 3 or 4 am so she'll come sleep with me and snuggle up to me, she says it comforts her, but she snuggles very close and i can't keep my hands to myself if you know what i mean. But i feel like a complete **** i don't wanna do this to my brother but i swear i love this girl, what do i do please?