What reason is your co-worker giving for changing her booked maternity leave and how pregnant is she?
What do your contract/handbook say about employers being allowed to cancel leave?
The reason I ask is that there are rules about notifying maternity leave start dates and the notice that must be given to change this, unless she's signed off sick in the last few weeks of pregnancy. There are also general rules about amount of notice that employer must give to cancel annual leave. Both of these are usually spelt out in detail in your T&C's.
Given that she's moving it forward by two months, I'm suspecting that she's not in that last few weeks. It changes the picture, because - and I may be remembering incorrectly! - that should mean she needed to give 28 days notice in writing to change her dates, and the company therefore would have had plenty of time to resolve this issue without flapping at you had this been 'maternity leave'.
However, I think it more likely that your co-worker is struggling with her pregnancy and has been signed off, if she's leaving at such short notice. This is relevant you, because it means that what your employer is asking you to cover is a colleague sickness absence, not a 'maternity leave'.
The key issue there is when your holiday actually is and how much notice they're giving you about cancelling. If they're giving you the stated amount of notice, they may be right - on technical grounds - about being able to do this. They have a business reason in a key member of staff being unexpectedly absent, and that may be all they need. It's a bastard thing to do, no argument, but they may be able to. Most contracts do have that clause.
Under those rules, you would then be AWoL, and it would constitute Gross Misconduct. Is it the normal procedure at your workplace for you to over each other like this? If there's custom and practice for cancelling holidays at short notice, particularly if you and this lady have done it for each other, it may weaken your position when this comes to the crunch.
What I'd be inclined to do is read your contract and then write to HR/Senior management and explain the situation. Point out all your extenuating circumstances, and note that, if you are outside the notice period is that this should mean they have a good number of weeks to come up with another solution as well, as you would consider the nature of your father's illness/distance/financial implications to be exceptional.
If they aren't within that period, then it's honestly their problem and not yours, but be aware that it will cause resentment. Go, and prep your documents for the all out when you get back.
It's a shit situation and it sounds like your company seriously needs to look at its staffing and back up procedures, as it's poor management that this situation has been allowed to occur.