I have no clue about what it could mean.
「Phrase/Mini-Sentence + ものだと思っていた」
is probably better if you remembered it as a fixed expression meaning:
"I just took it for granted that (Phrase/Mini-Sentence)."
"I never had the slightest doubt that (Phrase/Mini-Sentence)."
It would be practically impossible to translate the 「もの」 all by itself here, but it is being used to refer to an "assumed fact".
"Without thinking, I simply had assumed that my older brother would sit down."