AIBU to not want to go to SIL wedding

[thisstuffalwayshappenstome]

Looking for advice.

SIL has decided to get married abroad with her partner paying for it all. His family are (luckily for them) able to afford to go.

Her family is the opposite. It is going to cost £1600-£2000 to go on a holiday for a week (to a country we do not want to particularly visit) and to go to her wedding.

She is fairly sensitive about her family and their views (the wedding has already been booked once before and everyone had a negative view).

AIBU? We are not in the best financial situation and we are trying to plan and pay for our own wedding in this country!

If you can't afford it you can't afford it. I would decline and tell her why. If it is SIL then it really is your DP's job to inform her.
Tell her you would love to celebrate when she is back and send a card and gift when she does get married

Not unreasonable at all. If people decide to marry abroad they know full well that it will mean financial and other stresses for guests and should be prepared for no one turning up.
I wouldn't feel guilty at all about it if I were you.

YANBU if you can't afford it. Just tell them that (as nicely as possible) and then how they react is up to them.

It sounds like there are two different issues going on here, she has chosen a wedding not everyone can afford to attend and she is concerned that her family don't like her fiancé. YANBU to not attend based on the expense but the problem is she will probably take it as a judgement on the relationship. All you can do is calmly make it clear that you can't afford to attend but wish them the best.