My friend invites me to her house all the time and I have never invited her to mine?

Your friend wouldn't ask you if she didn't want you over. And probably wouldn't ask you regularly if her parents weren't comfortable with you. So I don't thing you're outstaying your stay. It is curious that she always seems to have a reason to not come to your house. But, seriously, some people are just more comfortable in their own homes. So it may have nothing to do with you or your home at all.