With regards to your outfit, I would say something smart (I'm not sure if you mean formal as in evening wear or suits and hats) but something subtle and maybe not as attention grabbing as a traditional 'Mother of the Bride/Groom' outfit. However, this obviously depends on your relationship with the Groom and your role on the wedding day.

You have said that this is a formal wedding, therefore formal dress is appropriate. Please remember you are important, this is your stepson's wedding and you are the wife of the groom's father you have every right to be there and be counted. My advice to you is go for it! Get yourself a beautiful outfit. The only thing I would say is try and find out what colour the bride's mother will be wearing, as both of you do not want to turn up in the same colour. Also avoid buying from the high street, choose from an exclusive boutique there will be less chance of similar outfits. Get your make-up on, get your nails done and go strut your stuff!

You should still get dressed up but as a matter of courtesy check with the groom & brides mothers what they will be wearing so as not to buy the same style and colour.