Emma,
hate to sound harsh but.....im not going to look at what they look like as in my opinion its obviousley not what you want and why should it be after you spending the time to design your own. Its your wedding day and part of the enjoyment of planning is doing things how you want to and picking and choosing colours, designs, fabrics etc as you go along. To have the part of you designing your own invites taken away from you would be terrible i think as youll always be thinking to yourself 'mine would have looked better' which by all means from the impression im getting off your thread is what will happen.
I really dont mean to be upsetting you here at all but you did ask for an honest opinion! I would stick to my guns if i were you, how about letting step mum do something lese a bit of a smaller importance to yourself (maybe she can have the rsvp's going to her house) or let her organise something else you havent got around to doing and arent too bothered about doing??
hope this helps and if not hope ive not upset you in any way
collettexx