You do not need your Inlaws permission to go to your friends wedding, or to live your life the way you want to. @Watdidusay Your in laws will make you feel like a "Horrible" person?
So they've done that before then?
I can tell you they will again.
If not this, it will be something else. DH will say you have to give in. I bet Christmas is fun in your family set up.
OP I look back and really resent the fact that certain relatives thought they could threaten us (usually me) with anger. That's what it is in reality, a threat. Obey or face our wrath. You are not children. You are not their child. In fact it sounds like you are already in the exclusion zone. Of course we should respect our elders, but can you really respect people who control by threat? If they really cared about you or DH they wouldn't spend their time making you feel horrible. Pity them instead for the way they choose to live their lives.
DH under pressure too.. control your wife... and is probably relieved in some ways that you are the target for all the flack. It was probably aimed at him before you came along. Eventually, I decided that since i was always wrong whatever I did, there wasn't much difference in their response if I did what suited our own family unit best. No is a very powerful word, but you have to keep saying it and meaning it. Guess what. Once they realised it wasn't happening whatever they said. They had to put up with it (until the next time) Try it.
You know they are being very unreasonable. but you also know that you are in the right. So don't let them. Yes, you will face some huff.. But. So what. Let them be angry. What are they going to do about it apart from say some nasty things. They already do that. Answer back. "What a horrible thing to say." Call it exactly what it is. "Is that a question or are you just enjoying ranting at me?"
Grey rock it, remain serene and calm as if all is well and as it should be... a careless cheerful "oh well, it's done now. Let's not dwell on the past!" Once they see their huff is having no effect, it will take the wind out of their sails. I promise you, this works.
I'd suggest that you don't have much to lose but you do have a chance to know that you are doing the right thing. Your friend will appreciate it. So many have put forward the sensible suggestion, you go to friends and DH to SIl.
Talk to SIL she may understand. Suggest celebrating with her afterwards and get her a nice wedding present. And get DH to back you up. Teach him to say "It's been settled. That's enough."
Your friend will understand that you are going but he "can't".
When you are older and your children have grown up you will be glad that you struck out for independence and didn't just end up doing things you knew were not right.
Do you have your own family for support?