My HVM has an interest in my teenage sister, is this a cause for concern or am I being paranoid?

So I have been seeing this supposedly HVM for about a year and he knows I have 3 siblings(2 bro, 1 sis) but tends to only ever ask me about my 17 year old sister (i.e. does she have a boyfriend yet, has she drank alcohol, asks to see a picture) are these questions normal behavior? I honestly feel sick to my stomach even thinking about him potentially using me to attempt to initiate a relationship with my sister. I will be completely honest and say that sometimes we will engage in light conversation whilst doing regular foreplay like activities and he was asking me about whether or not my sister has had a boyfriend whilst he was erect. Should I block/delete? or am I just being paranoid.