@Aprilcherry04
The thing is he does know his cousin very well and he also does understand what a wedding is. I know that my DS1 will also be offended that his brother has not been invited and probably not attend either and this will create a family rift.
I would, frankly, not go. I know it's difficult but it is quite offensive that your youngest son is not invited. What is wrong with him dancing with or without girls? I presume he doesn't do anything untoward. He will have his dad and brother there to make sure he doesn't become too boisterous and if as you say he likes his cousin and knows about weddings, he can be told to reign it in - which might not be necessary.
There are often embarrassing people at weddings who don't have learning difficulties! I remember a wedding where a grandmother had too much to drink, collapsed on the floor and had to be dragged out of the room :-). She was too heavy to pick up. Often there are men who drink who are suggestive with women, and girls who act daft. When I was child, one of my cousin's husbands got drunk and objectionable at a family wedding, drove home would you believe and crashed the car. Thankfully nobody was hurt.
Your son won't do any of that!
The days of banishing people with disabilities should be behind us. I am appalled that it was even suggested but it was obviously not your sister in law's or her son's idea.
Just don't go, all of you; I'm with your husband on that. Even if they change their minds, it's been said. I'm not surprised you have been crying, I am appalled on your behalf.