I am in a boot for a broken bone in my foot. Wearing a boot made my spondydislothesis, which hadn't been a problem, cause sciatica. Help!?

Spondylolisthesis. Was always there long before the boot was on your foot. Likely the change in your gait caused it to pinch the nerve exiting out of that level. I would suggest you see a pain/spine fellowship trained specialist. You may be a candidate for an epidural steroid injection.
Yes. Anyuthing which changes the angle of yor walking can create extra stress on your pelvis and spine. You can try some simple stretches like williams low back exercises, don't stand or walk too long, and see if it will relieve your symptoms. If not and it persists, see your family md for advice on how to help treat your symptoms.
Nerve block. Limping is hard on low baclk get out of boot get a selective nerve root block leg pain will calm down.