Is this fair?

Hell nah that's what the deposit is for! And this poor woman is going through such turmoil how awful of them to try and take advantage of her misery.

This. Texting other men, just before you are supposed to be wed, is grim.

Nah, she's sabotaged her wedding, nobody else. Personally I'd still go if there were other people in the party who fancied it, but if not, there's not a chance in hell I'd be losing money on this. No way.

I would go to the weekend abroad anyway with my mates if I were her.

Either they still go or they all get the £300 they have already paid back
Why should they all lose money - even if she wasn't the reason the wedding was called off they should all get the money back - but the fact she was cheating she should be falling over herself to pay it back.
Agree with everyone else - if you cancel you lose your deposit - you don't pay anything more - she is trying to recoup money for other deposits she has lost - guessing her fiance is making her foot the bill for the cancellation

Your dd should flip it and ask for the return of her £300 since your niece has cancelled the wedding, and by extension the holiday.

Exactly and as someone has already pointed out, she must have money saved for the wedding that she can use

Has the bride already cancelled without informing everyone? There's no way she'd be getting any more money out of me for her f*ck up.

I'd just go on the holiday rather than lose £300/400.

Exactly what I was thinking. I know the op says there's a cancellation fee but personally I'd be telling the holiday company to do one and asking WHEN they resell the holiday to refund the deposit. Pretty sure that given the time frame, it is actually a legal precedent that if the holiday is resold, then the holiday company/hotel/airline hasn't lost out and should refund the deposit less a reasonable handling charge...

Mind you is she the one who called it off?

She should ask whether people would like to still go as a holiday (easier if everyone knows each other and isn't just connected via her), and if not, I think it probably needs to come out of any money saved from cancelling the wedding.
If only some want to make the trip anyway, they may have other friends who could fill the spaces.

Why would the family be annoyed with your daughter rather than the non bride?

I think your daughter should still go . If she is bride's sister friend , what is the problem? Rooms already allocated. It's not going to be a week is it ? Why would the family be upset with anyone except the Runaway Bride ?

How much was the total holiday OP? I would be inclined to go rather than lose £400. Especially if it's All Inclusive and not much spending money needed.

My first thought on reading the OP too. This makes no sense

The bride should really be covering everyone’s lost costs.

Sorry.. should have said: "and yet it is your daughter who is worried" (not niece - who is the CF in this situation)

I think everyone else has said it all really. If the holiday is in her name and she organised it and she screwed it all up due to her immoral actions then she should pay the cancellation fee. I think everyone accepts if they have to pay a deposit there is a chance they might not get it back due to unforeseen circumstances (illness, weather, cheating brides etc.) but she cancelled the wedding due to her actions so she pays the cancellation fee.

But where were they going that the deposit was £300 each? How many of them were there?!

It’s the (not actually) bride’s problem and fault so she can pay it. I’m a soft touch and a pushover and even I wouldn’t pay the £100.

Not a chance would I pay it & I'd be asking for my deposit back as well. She has to take responsibility for her actions which have lead to wasting people's money. What an absolute liberty.

Wow. The ex bride is a massive CF.

She took a risk booking the hen do abroad and asking her friends and family to cover their own flight and accommodation costs so the least she can do is cover the cancellation fee.

I think my daughter will have to pay it as she doesn’t want to upset the family.
The brides family is more important than your daughter and they get to choose how her money is spent?

I wouldn’t be paying £400 for a non-holiday!

Oh, tricky.
I think if it is a poor bridesmaid stuck liable for the bill I’d pay it, begrudgingly.

If it’s the bride on the hook, well she’s made her own bed…
Agree it’s a good idea to turn it into a girls holiday.

I wouldn't pay and I'd be expecting the £300 back as well, since it's her cheating that is to blame.

The un-bride was texting other men, which caused the cancellation.
Un-bride is asking people to pay £100 on top of the £300 they've already lost. So, every will be £400 down entirely to due un-bride's actions.

And yet it's your niece who is worried about family upset?

Er, nope.
(Un-bride is a CF and needs to cover the cost herself and not mention anything more about this entirely avoidable self-inflicted problem she has brought upon herself)