Family issues

Hello ladies, I would like your advise please. My Dad and his wife have a teenage son and from the very start of the planning process my Dads wife has been very vocal & pushy that he should be an usher. It was almost that it was expected/assumed. Neither my partner or I want him to be an usher so we politely told my Dad that my partner has picked his close friends only as his ushers and we don't have any jobs for him to do but we're happy to buy him the same suit as the ushers etc so that he feels part of the bridal party. My Dad has given us some money towards the wedding so I thought that this was a fair compromise. So a month or so has passed and I thought all was fine but his wife is now talking about sitting at the head table with us, welcoming guests, coming with my Dad to collect me etc....now you may think just let it happen it's not the end of the world but unfortunately he has a bad attitude, temper and foul language his parents are very defensive of his behaviour and make excuses for it so it's hard to bring up) so I don't want him involved. How do I make this stop without upsetting people?