It's a security thing...she feels most secure on your lap...with her blanket...sucking on the blanket...with happy feet. Not a thing in the world wrong with it and it's probably made your bond insanely strong. Besides, it's a very calming thing, it probably winds you both down from the days events.

No offence meant but who's your sister to say how your relationship should go with a kitty that's obviously yours through and through?

I say that you should let your sister keep hating it...surely she could find something better to do than complain, but then, maybe she's jealous deep down inside.