You need to find other people/things to place your attention on. I don’t think it’s as bad as you make it out to be, there is no blood relation and based on your circumstances it makes sense that you would have these feelings towards a girl you see who is about your age and attractive. I would stop making excuses for why you can’t meet other girls or have other friends, until you’ve committed yourself to at least a year of trying new ways. Put yourself out there more to meet more people. Join a new group that does something your interested in like a sport or a hobby that can be done with multiple people. The bigger problem here is that you don’t have people to talk to or potentially date and so you’re placing all your eggs in one basket, and it’s an inappropriate basket (but understandable). Set goals and dedicate yourself to having more friends as you enter adulthood. This will help you both personally and with the feelings you have. If you have friends it will become easier to date. If you did someone else you’re attracted to, it will be easier not to obsess over this crush and accept its not going anywhere