To late now to fix this, should have been taken into account from the begining. If you dont want railing encloached can use post bases and put the posts on the side of porch but he will have to make concrete surface to mount the post brackets. Can use aluminim railing if the vinyl railing can be returned, its not as thick and it will encroach less.

Can the stairs be expanded by another 4inch on both sides now to accomodate the railings?

Can the stairs be expanded now,4inches on each side, to accomidate the railing? would it keep the structure stable or make it weak?

Railings don’t have to be mounted ON the step surface. (Of course, it’s easier if they are.)

Thank you for the answers but can the stairs be extended on both sides now to make them wider.