YOU SAID:

I was walking down stairs and then I saw my brother playing video games so I joined and played some video games with him

INTO JAPANESE

私は階段を歩いていました

BACK INTO ENGLISH

I was walking up the stairs

INTO JAPANESE

階段を上っていました

BACK INTO ENGLISH

Was climbing the stairs

INTO JAPANESE

階段を登っていた

BACK INTO ENGLISH

Was climbing the stairs