Re: brother and his irritating girlfriend
Maybe there is a reason your brother felt more comfortable introducing his girlfriend to your younger sister rather then you? Maybe he feels its time she got to know his family since he wants to marry her? Maybe she is wearing hijab because she realizes the importance of religion and wants to get married to your brother and live a good life? Maybe its time for you to be there for him rather then against him? Maybe you should make this ordeal a little easier for your parents by helping them understand where your brother is coming from rather then ignite them against him.
See, you can either keep your family together or tear them apart. Which will you choose?
My advice:
Talk to your brother, advise him to keep your sister out of it. Other then that, its not your battle and you have no business getting in the middle of it.