Thanks.

I don;t know how many errands I had done for them.  They don't normally ask my sister since she would just say no.  Every time after something is done, they would tell me "We could have done it without you." 
This has gone for more than six years now.  So i have done some self-reflections.  Is that worthwhile to be nice to selfish parents?  My time and energy is limited.  I spent time for them, and that is going to decrease the responsibility I have for my kid.