I think I love my sister in law

My question is this. I need some serious advice and it's a very tricky situation and subject. I met my girlfriend almost 6 years ago we fell in love the rest is history but this is where the plot thickens. I have a sister in law yes you guessed it!!!! When we first started seeing each other my sister in law was constantly critiquing me and slagging me off behind our back about all and sundry. Now the situation is this. I knew this was going on and decided that I would make friends with her for the families sake and started to help her around her house the usual diy stuff as her husband as nice as he is is useless at it. We have started texting and messaging and recently we have been having a laugh. The problem is I've started to notice that she is wanting more contact. Ie yesterday and today we have been texting about all sorts but I paid her a random comment and she's wanting to know more about how I feel etc. I wouldn't say she's the most attractive woman on the planet but I'll be honest I wouldn't turn her away in the right situation and this is where I'm concerned. For some reason I've started to have feelings for her and don't nniwbwhat to do. I don't know what to think say or even feel. Yes she turns me on and yes she makes me feel good even but the smallest of stupid things. I love me girlfriend more than anything but it's driving me nuts I can't get her sister out of my mind. I don't think anything will ever happen between me and the sister In law but it's playingon my mind constantly. I just want to know what she might be thinking. Is tis just me reading into things or is there something there. Someone help me please. If you want more intricate details the. Please fire away