Can an Unranked play with a Plat?

then how can it be that i play with plat on my brothers account?

In a normal, Yes. in ranked, no

Kk thanks, do you happen to know what the limit of unranked and ranked has?

Unranked players who've never Ranked before are considered Silver for queuing purposes. If they are simply Unranked because they haven't done placements this season but were Ranked last season, they'll be considered where they were then (so if they were Plat last season they queue by Plat rules). If a player did not Rank the prior season or had their rewards stripped they'll be considered Unranked.

https://support.riotgames.com/hc/en-us/articles/204010760-Ranked-Play-FAQ#h4q2