<human>: My brother proposed to his long-time girlfriend about six months ago. Her sister has always had a thing for me. We never really talked until last night at my sister's 30th birthday celebration. She ends up being really cool, and we manage to separate ourselves from the group to make out. We take things to my car and things get a little more physical. As all of this is happening, her sister is calling her and my brother is calling me. We ignore our phone calls. When we walk back to the bar, my brother is outside waiting with a grin on his face. 

He tells me not to feel weird. "I'm proud of you" he says. He tells me that his fiance's sister just got out of a long-term relationship and to proceed with caution basically. He seems to be totally fine with me seeing his fiance's sister, but I don't know how I even feel about it.

Even though we're not technically blood related, I'm worried about what our families would think if we started dating. There's also the awkwardness that would come between us if things didn't end amicably. I might be overthinking this, but this is a unique situation that I could really use some advice on.

Give me a shortened version of the text
<bot>: I hooked up with and started developing feelings for my future sister-in-law's sister last night. I'm not sure if it's a good idea to start seeing her.