Re: How fair should one expect in laws to be between daughter in laws?
Since you are at the early stages of OMG-WHY-MY-INLWAS-ARE-NOT-RUDE-TO-ME-I-SHOULD-FIND-SOMETHING-syndrom, Id advise that you keep these thought out of your mind and shut the door for these thoughts.
you dont know their relationship with your sister in law. It might have happened that their relationship with her was more developed at the time of her Nikkah than with you. There can be any reason or may be she is just her favourite. As long as they are not treating you bad, they are good in-laws.
Its good you got into arranged marriage warna aap k tu baladkar he ho jata 