Why do people think they have a right to use people when they want something?

it seem like my sister only wants to know me when there is something she wants like babysitting or looking after her house when she goes on holiday then she comes to me practically every week but when she has what she wanted i don't see her or the kids sometimes for months she only lives 20 minutes away and she drives i don't when ive mentioned this to her before she either turns it into a big argument or she will apologise but within a few months she is doing the same thing again if it was a friend treeting me like that i would drop them like a stone but she is family and even if i can live my life without her i would'nt be able to see the kids i dont think i could cope with that