It is a few hundred meters long.
If you and your sister live along the street on opposite side, you would have to carry your passport and report to custom every time one of you go to see the other. Until 9/11, they used to simply wave the local residents, but not anymore.
Both customs offices are facing each other at the end of the Street
My memory is not that good. Border stations will fly their flags at half mast if former presidents or members of congress die, or if an INS or Customs officer is killed in the line of duty. Also state governors can call for the lowering of flags.
Yes, the border is straight north to south and clearly marked by the international boundary commission. Cleared, monuments to let you know you are in Canada or US.