I seem to be in the minority but I find the whole "stepbrother" plot "twist" too ridiculous even for a Bond film. So much so that I'm afraid it fundamentally damages the whole film for me. I was therefore going to just ignore that whole story line (much like the same way we can ignore Bond previously meeting Blofeld in YOLT when watching OHMSS). I think this actually improves my enjoyment of Spectre as it would explain why the storyline has no plot implications (eg Bond does not react when told his "father figure" was murdered and he even uses the name Blofeld in the finale when logically he should call him Franz - if only to piss him off).
However I think it may just be possible for us to take ther view that Blofeld is not Franz and was just lying as part of the whole torture process in order to get to Bond. Admittedly I have only seen the film once so this could all fall to pieces But Bond only initially sees Franz in shadow and do we actually see any evidence that Blofeld is Franz (other than the photo of course)? Is it concievable that Blofeld found out about Bond's investigations into the death/faked-death of Franz through his spying on MI6, put two and two together, and used this information to lie about being Franz in order to try to really get to Bond. In fact this could extended to cover the whole "author of all your misfortunes": Blofeld only tells Bond this, we never see any evidence.
I know this all sounds a bit silly but surely it is no more ridiculous than asking us to believe that an annoymous ski instructor was a father figure to the world's best hero and the world's worst bad guy? Anyway it works better for me as I think the actual plot point diminishes Blofeld by making him a madman with Daddy issues.