Skinship between a mid-30s uncle and his 5th-grade niece

I’ve been dating my boyfriend for about 3 years. He’s in his mid-30s and has lived with his sister's kids since they were born. Currently, the household includes a 4th-grade nephew, a 5th-grade niece, my boyfriend’s mom, and him. The sister is divorced and working in another city to make money, though she still takes good care of the kids. When I first met them 3 years ago, the niece was in 3rd grade and was very pretty. I saw my boyfriend’s photo album and was a bit shocked to see photos of them kissing on the lips. I have three nieces/nephews myself, but I’ve never kissed them on the lips unless they were babies...