The entire ranked system for arena seems to be quite broken TBH, maybe much due to how relatively unpopular the mode is among the Apex player base. I think it messes up the individual ELO vs rank system when the player ELO distribution isnt "evenly" distributed etc.
But in your case, perhaps your cousin had a slightly higher individual ELO than you, coming into the placement matches? There is alot of stuff under the hood of every system that isnt that obvious. I played very few games of arena when ranked was released and ended up in Diamond and the next split I played arena (dont even know if it was one or two splits in between) , I performed quite average in the placement matches but ended up in low Plat.
