The woman, who is getting married this year, explained how her sister - who is her maid-of-honour - 'took on the responsibility of planning [her] hen do'.
Despite never actually discussing any available dates between them, the maid-of-honour went ahead and booked a surprise trip away.
The bride then got a text from her maid-of-honour saying a week long trip was booked and paid for, so asked for the dates so she could book the time off work.
When the bride-to-be submitted a holiday request off for the organised dates, her boss told her that 'someone had already booked that week off and [she] couldn't have it off', she said on Mumsnet. But the only response she received from her sister was to 'call in sick'.
Despite the situation, the bride's maid-of-honour didn't make any attempt to change or cancel the tickets, but instead continues to organise activities and has said that she's still going on the trip whether the bride goes or not.
Asking for advice and input from the Mumsnet users, the bride-to-be asked if she's being unreasonable 'to think this is totally ridiculous and [her sister] really just fancied a week holiday?'
After posting the thread, the bride-to-be has received many sympathetic comments in response, empathising with her situation.
'That is totally ridiculous - you need to tell your sister that it's fine if she wants to just go off on a jolly but she still owes you a hen party! It's insane not to check the bride's availability before booking!' commented one.
'Just don't go and leave them to it. Who organises a week's holiday without checking availability of the main participant!! She sounds incredibly selfish' said another.
The bride-to-be went on to explain that she was especially upset because while it isn't a huge group going on the supposed 'hen' week away, the group includes her sister, mother, aunt and two closest friends from uni.
In a bid to help her out, several users suggested ways to overcome the situation, like trying to rearrange the trip.
'I'd ask for the details and call the company yourself to see about rearranging. The hen do is all about you, not at all about them. I'd refuse to go on a hen do if the bride wasn't going to be there', one said.
A second added: 'Your sister sounds really dumb to have booked this without checking with you and all the hens first. I'd be getting her to check the cancellation fees.'
While a third also said: 'I think you picked the wrong person to be your matron of honour, your sister is very selfish and self-centred. Why not let your other friend arrange a night out and tell everyone that is the official hen do?'