My wireless modem is getting fixed as it broke, so i am connected to someone in my street, is this illegal?

Well here's a comparison for you to see if you think it's legal or not. Supposing someone in your street had their car break down so they just come and took your car. Would that be legal? How would you feel about that happening? I know it's a different scenario but in principle it's exactly the same. I know that nobody can see you using someone else's broadband, but you are still stealing from that person. So yes, what you are doing is illegal.