Being on remission

Not an easy one to answer JS but obviously it's well known that RT can cause other cancers years down the line. 

There are good treatments for BC these days, on the plus side. Has the specialist indicated  a treatment plan?

Happy New Year

Unfortunately, if you've already had one cancer, then you're more likely to have another.  I know someone who's had 3!

It's largely due to.immunity, which is popularly seen as affecting resistance to infection.  It is, but more generally affects resistance to anything that's not recognised as "normal" e.g cancer cells..

I suppose passive smoking could affect your immune system and/or introduce some other effect on your bladder indirectly, but it's hard to imagine any direct effect.

Bladder cancer is treatable, whatever caused it is now possibly irrelevant.

If you've now given up passive smoking that's good.  If you haven't. then you may have to think about what you can do to avoid other further consequences.