Yours are mine,but mine still belongs to me.
Singular subject (you=1 person) = verb with S = IS
Plural subject (we=more than 1 person) = verb without S = ARE
We don't know what we are talking about here...
If one thing is mine then we can say 'Yours is mine"
If more than one thing "Yours are mine"
In this sentence maybe it is more than one thing...
so we should say
"Yours are mine,but mine still belong to me."
That computer is yours,but my computer still belongs to me.
Those computers are mine,but my computers still belong to me.