Yes, the behavior you described is excessive and inappropriate. Unless your step son has some kind of developmental disability, which you have said he doesn't, this stuff is over the line. Tying his shoes for him is odd, but the showering, drying off, and dressing help is really, really inappropriate. I understand that sometimes divorced parents over-do for their kids in some way out of guilt. I also get that desire to want to keep your youngest a "baby" for as long as possible. At 12, this young man needs to take care of his own showering and dressing - end of story. Someone mentioned this and I'd like to say it again, what happens in a few years when he turns 18, goes to college and has never even dried himself off or dressed himself?
It sounds like your husband has some repair work to do with his relationship with his daughter and my advice there is to do it while she still has a desire to repair the relationship, since soon there is a chance that she is going to be too hurt and shut him out.
Being the step parent, this is a delicate situation for you. Did the kids or your husband do any kind of counseling during or after the divorce? If so, it would make sense to go back in for a few sessions (if they felt that person was helpful) or to find a family counselor to support the independence issues and healing the relationship with his daughter. Good luck to all of you.