We're having the same problem with my husband's family - he has many brothers and sisters he's never even met, so doesn't know their birthdays let alone addresses or birthplaces... Same goes with his parents as he didn't really grow up with them either and both are deceased (so don't know date of death required for the IMM0008 form either). it's pretty confusing really, i haven't come close to figuring out his family tree yet.
Anyway, I'm hoping that we can just fill it in as best as possible. He's tried looking for birth certificates - but they can't find them without the birthday!
I did ask the CIC contact center - and they said just fill out as best you can and write a letter explaining... that seemed to be the generic answer for all of my questions actually!
Any extra input on this would be good (any suggestions on how we can track down this info?)...