No Potters?

I am listening to Harry Potter and The Order of The Phoenix for the umpteenth time. My least favorite of all the books!! But more than once I have wondered why there are no Potters. Sirius mentions to Harry that he ran away from home around the age of 15 or 16 and that James Potter's parents took him in. A perfect opportunity for Harry to ask questions about his family. After all they are looking at the Black Family tree at the time. The Potters were a pure blood family, and Sirius says all of the pure blood families are connected in some way. It strikes me as, well Stupid, for Harry not to ask about his family tree in this scene. What happened to his Grandparents? He has never even asked if his Father had brothers, sisters or cousins. But when he looks into the mirror of erised he sees lots of Potters and his Evans relatives. Another stupidly skipped opportunity - why did he not ask Dumbledore what happened to all those relatives he sees in the mirror? Hagrid recited to Harry a long list of "powerful wizards and witches" of the age destroyed by Voldemort and his supporters, he never mentions any of the other Potters or their spouses. There has to be a reason that Petunia Evans Dursley is the only one left - why didn't Harry ever wonder WHY???