We have two questions: (1) How do you include a stepfather who has pretty much supported the bride financially and mentally and the bio father in giving away the bride whom she is close to but he just doesn't provide for her and (2) how do you include a son from a previous relationship who is 2 yrs old and a 1 yr old daughter from this relationship in the wedding?

You can word the invitations from your step-father and mother (who are probably paying for the wedding). Your step-father can walk you down the aisle, or your bio father could, or both of them together can walk you down. You could also have the first dance with your bio father and have your step father do everything else. Or one of them could do a reading during the ceremony.

The children are pretty young to have any sort of active role in the wedding process. You could dress them cutely (a small tux and very small white lacy dress). It's probably a good idea to enlist a baby sitter to watch them during the ceremony and make arrangements for them after too (they will probably not be awake during the whole reception, nor would you want them to be, as they will need naps or to get to bed early). You can list them in the wedding program and include them in wedding photos.