They resent him divorcing their mother when they were children and his ex wife despises him
It seems to me that your dh is going to have summon a lot of courage to attend this wedding and act as if there's been no rancour between him and the fomer wife wife who "despises him" and the adult dc who "resent him" for divorcing her and, far from accusing him of disloyalty, you should be supporting him every inch of the way in going along with the wishes of his son.
If "the shoe was on the other foot" I would think you were unhinged wholly and thoroughly unreasonable not to attend your own ds's wedding if your new partner wasn't given pride of place. What kind of narcissistic mother would do that to her child on the child's wedding day?
Has it occurred to you that the wedding has been arranged in this particular manner for reasons entirely unconnected with you? It may be that bride's dps are also divorced and/or the number of guests at the celebratory lunch has been curtailed for reasons of cost - inviting you to attend would mean paying for an additional 4 meals as it would be unfair to leave your dc out.
As for the invitation to the evening event addressed to your sole name; this is perfectly correct etiquette as practised at Buck House and it is not intended as a slight. Your dh has an invitation to the day and evening events, while you and your dc are cordially invited to attend the evening event.
What is really going on in your mind? Are you coming from a place of such irrational jealousy and suspicion that you imagine your dh and his ex will fall into each others' arms over the wedding cake if you're not there to police him?
sadly for our relationship I feel unable to forgive him for what I believe is disloyalty and don't want his sons to have anything to do with my sons if you persist in maintaining this negative attitude you shouldn't be surprised if your dh begins to emotionally withdraw from you and that 12 years of happy marriage is all you have with him as the remainder will be tainted by your unrelenting bitterness.
Give yourself a good shake; be glad that your dh is an honourable man and make sure you do nothing to upset his equilibrium or that of his adult dc.
I have no doubt that after an arduous day of putting on a good front, your dh will be immensely relieved to see you at the evening event; why not buy a beautifully boxed horsehoe and one of these www.ebay.co.uk/itm/Yankee-Candle-Wedding-Day-Large-Jar/121794475599?_trksid=p2045573.c100505.m3226&_trkparms=aid%3D555014%26algo%3DPL.DEFAULT%26ao%3D1%26asc%3D38661%26meid%3Da62605aa19c24ef68d643bef79f596f9%26pid%3D100505%26rk%3D1%26rkt%3D1%26 for the bride and tell her you hope that she enjoyed her registry office wedding as much as you did yours and wish her and her groom a long and happy marriage.