Is it normal that i'm crushing on my girlfriends baby sis?

I've grown a liking to my gf's little sis. I've been with my gf for 5 years with some time off cuz she went bag to her sleazy ex. I've gotten close to her family but mainly her and her little sister who I've know through out the entire relationship. Together we've gone through tough times and they came to me for protection. Throughout the years things have gotten better and I've gotten even closer to her sister we joke around laugh cuddle and watch movies etc. but lately it's different we get close just me and her she'll lay with her back to me my arms wrapped around her waist or she'll press up against my chest as I hold her and she'll start to kiss me all over my face and lick and bite my face(playfully).I try to distance myself from her but it's hard since I see her everyday. She's started to grow up more and it's impossible for me to see her as a kid anymore I've tried. She's grown into unfortunately my type and it's hard to ignore we even joke about it and she tosses the fact in my face. We recently went costume shopping for Halloween and I was with my girlfriend and she with her boyfriend and when they turned around me and her were talking and she went to kiss my face (we all joke around and kiss our faces) but this time she kissed my lips and looked up at me waiting for me to react. I didn't. I texted her later on and told her I know she gets nervous and she made a mistake but I'm not mad and she told me "I wasn't scared to do it" she's 14...I'm 20 how am I supposed to handle this It's making a pit in my stomach feeling this way I feel guilty and odd but another part of me doesn't. Is it normal?