I came across this task during my exam. 

> 
> Ted Sanford was born nine minutes later than his brother Harry. It was his twin brother, Harry, who was (**supposed/thought**) to become an earl, to inherit a castle in Scotland.
> 
> 
> 

I had a feeling that **supposed** was the right answer and it was indeed, but I can't explain to myself why **thought** isn't correct. Could you, please, help me with that?

I think this is in passive voice because here it is said that constructions with 'supposed' belong to Passive Voice. <https://learn-english.wonderhowto.com/how-to/use-passive-voice-supposed-english-234202/>