Boyfriend says im too close to his sister?

ive been dating my boyfriend for almost 10 months now and he introduced me to his family very early on in the relationship and i grew very close to his eldest sister.. i used to go down his sisters house like once a week we grew very close with each other mainly because she's going thru a tough time in her marriage and she asked me to build back her confidence and i said ok?.. my boyfriend felt very uncomfortable with this and he said he felt liek we didnt have a private life anymore..and tha he felt tha eveyrtime he did kiss me he felt as if he was betraying his family's trust? he feels that he's going out with his sisters friend and it feels wrong and i understand where he's coming from so i said i'd back away a little from your family buh he's sayin no my sister is involved now and if you drift away and hurt her i;ll never forgive you..i dont know what to do because he says he loves me his feelings havent changed and we both want to be together buh he keeps saying i cant do it.. i dont want us to end this is the most serious relationship ive ever been in but he just wants to be best friends now :S.. i dont know what to do anymore help :*