Being forced to go on stag do

My little sister is getting married in June and my future brother in laws stag do is planned for Madrid on Easter weekend. When the stag do was first mentioned last year I decided there and then I wouldn’t be going as because 1. It’s too expensive 2. It’ll be three weeks before my little boys first birthday and 3. I just don’t want to go anyway. My future brother in law is 8 years younger than me and we have absolutely nothing in common. Whilst he’s still into going out and drinking himself into oblivion I long out grew that.
My parents came round this morning and the topic of the stag do came up. I just told them I’m not going because I can’t afford it but they just failed to understand my point of view. To the point they basically said they’ll pay for me to go to which I said don’t waste your money on something I don’t want to do but they just ignored me. So now I’m being forced to go on this stag do when it’s going to be expensive when I’m there and both me and my girlfriend will have to take time off work so that I can go on this stag do and so that my girlfriend can stay off work to look after him.
What should I do? I really really don’t want to go but have essentially been forced to go by my parents. They won’t accept no for an answer and won’t respect me for saying the money I’ll spend in Madrid could be better spent going towards a family holiday for me, my girlfriend and our little boy.

Well first of all you tried to address it but in doing so you buried the lead. You suggested your reasons for not going was financial rather than the truth. The real bottom line is you don't want to and you should of started with that

It sounds like your role in going, from your sister's and parent's point of view is to observe proceedings.
And to report back if your possible future brother in law gets up to anything too outlandish.
That's an important role. And one where your sister and parents may not be able to turn to anyone else to do this.
You could even act as an agent provocateur. And encourage him to do stuff, "Don't be boring!!", and see if he bites.
Or they may be looking for you to be a calming influence on him, "Nah, that's a bad idea, let's do this instead!!"
Sometimes doing the right thing and stepping up to the plate involves you going the extra mile for the people you love.

They are both reasons I don’t want to go. I don’t want to go in the first place and I don’t really have the free money.

At the end of the day they’re not going to frogmarch you onto a plane with a hood over your head.

That is basically what is going to happen. My dad has said he is paying for me to go so now I have to go apparently. This however has now opened a whole can of worms where my parents are saying I am out of order for not wanting to go and I have told them they are out of order for being so controlling.

Stand up for yourself, say No.
If it was a local pub crawl or something I'd say just suck it up for sake of a night - but a holiday to Madrid, when you don't want to go? **** that!
Why is your dad going anyway? does he actually want to?

At the end of the day you have to decide whether your preference for not going is going to cause more hassle than it's worth. I'm going to guess there's some mad Asian/South Asian family politics going on here?
Personally I'm quite stubborn so would just ensure that they knew full well that I wasn't going and would then be someplace else on the day if they decided to arrive to 'collect' me.

I honestly am starting to feel like not going is more hassle than it's worth so I may as well just go and not enjoy myself because at least then I can say I went.

I don't know your family so do not know the dynamic but you have every right to say sorry you cannot make it.
You have to put your own young family first and it is not like you are best man.It does seem you have a bit of resentment about your sister not visiting your kids,is that playing in here?
End of day I would talk to your sister and ask if she minds if you do not go or just make your apologies and buy them a nice wedding present.
Husband just makes excuses now as he has grown out of all the drinking etc and costs a fortune

My dad doesn't want to go either but he says he has to go really because he is the father of the bride. So I feel like I'm being made to go just because he has to go.

Is that really a tradition/expectation?
I'm not married but was under the impression a stag night was for the groom, best man and close friends? I don't think inviting future parents in law is common?

My dad seems to think it is an expectation that he goes.