2 Answered the actual question.
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I guess the question you have to ask is, Is the journey that important to you?

You've got two weeks of Couples Time ahead of you and functionally 18 hours with Momfriend and Girlfriend if you do this.

This is an opportunity for bonding time with Momfriend and even if you spend the entire journey in silence (very doubtful given the personalities you describe) it's a favour that absolutely will ingratiate you to Momfriend.

It might be a lot of fun, it might not, but being precious about plans is rarely a good thing.

I speak from experience on that last point, I have a borderline pathological hatred of sudden changes of plan and really relate to how you're feeling on the subject.

To decide, maybe ask your girlfriend what her mom is like on long car journeys, if they've been on road-trips before, does mom sing? is the conversation stimulating? Is she awful to travel with? Motionsickness?

If she'll be a good travel companion then your journey might be a lot of fun.

On the other hand, That "we're on our way" feeling when you set off on your own together really detaches you from normal life a lot and travelling with Momfriend might well disrupt that, but it's a fleeting feeling, it'll be gone within half an hour or so. So the question is, Is that feeling really worthwhile?

Edit:

If you're set on refusing however, I would elect to be frank about it. Explain that it's important to you that this be 100% couples time from start to finish. Be apologetic but firm.

Momfriend ought to understand it, you might leave her in the lurch as far as travel goes, but she is imposing on your trip and you have every right to refuse. People are not always reasonable however. Very much depending on the individual, you may not have a way to refuse without upsetting anyone.

Talk to your girlfriend about it, she knows her mom best and given the request is coming through her rather than directly, she's probably going to be the one to deliver the bad news to mom anyway.

I also really cannot stress hard enough that you should make sure you and your girlfriend are on the same page about this.
Her priorities may not be aligned entirely with yours and being too firm about refusing may put a damper on the trip.

I guess the question you have to ask is, Is the journey that important to you?

You've got two weeks of Couples Time ahead of you and functionally 18 hours with Momfriend and Girlfriend if you do this.

This is an opportunity for bonding time with Momfriend and even if you spend the entire journey in silence (very doubtful given the personalities you describe) it's a favour that absolutely will ingratiate you to Momfriend.

It might be a lot of fun, it might not, but being precious about plans is rarely a good thing.

I speak from experience on that last point, I have a borderline pathological hatred of sudden changes of plan and really relate to how you're feeling on the subject.

To decide, maybe ask your girlfriend what her mom is like on long car journeys, if they've been on road-trips before, does mom sing? is the conversation stimulating? Is she awful to travel with? Motionsickness?

If she'll be a good travel companion then your journey might be a lot of fun.

On the other hand, That "we're on our way" feeling when you set off on your own together really detaches you from normal life a lot and travelling with Momfriend might well disrupt that, but it's a fleeting feeling, it'll be gone within half an hour or so. So the question is, Is that feeling really worthwhile?

I guess the question you have to ask is, Is the journey that important to you?

You've got two weeks of Couples Time ahead of you and functionally 18 hours with Momfriend and Girlfriend if you do this.

This is an opportunity for bonding time with Momfriend and even if you spend the entire journey in silence (very doubtful given the personalities you describe) it's a favour that absolutely will ingratiate you to Momfriend.

It might be a lot of fun, it might not, but being precious about plans is rarely a good thing.

I speak from experience on that last point, I have a borderline pathological hatred of sudden changes of plan and really relate to how you're feeling on the subject.

To decide, maybe ask your girlfriend what her mom is like on long car journeys, if they've been on road-trips before, does mom sing? is the conversation stimulating? Is she awful to travel with? Motionsickness?

If she'll be a good travel companion then your journey might be a lot of fun.

On the other hand, That "we're on our way" feeling when you set off on your own together really detaches you from normal life a lot and travelling with Momfriend might well disrupt that, but it's a fleeting feeling, it'll be gone within half an hour or so. So the question is, Is that feeling really worthwhile?

Edit:

If you're set on refusing however, I would elect to be frank about it. Explain that it's important to you that this be 100% couples time from start to finish. Be apologetic but firm.

Momfriend ought to understand it, you might leave her in the lurch as far as travel goes, but she is imposing on your trip and you have every right to refuse. People are not always reasonable however. Very much depending on the individual, you may not have a way to refuse without upsetting anyone.

Talk to your girlfriend about it, she knows her mom best and given the request is coming through her rather than directly, she's probably going to be the one to deliver the bad news to mom anyway.

I also really cannot stress hard enough that you should make sure you and your girlfriend are on the same page about this.
Her priorities may not be aligned entirely with yours and being too firm about refusing may put a damper on the trip.

1
source | link

I guess the question you have to ask is, Is the journey that important to you?

You've got two weeks of Couples Time ahead of you and functionally 18 hours with Momfriend and Girlfriend if you do this.

This is an opportunity for bonding time with Momfriend and even if you spend the entire journey in silence (very doubtful given the personalities you describe) it's a favour that absolutely will ingratiate you to Momfriend.

It might be a lot of fun, it might not, but being precious about plans is rarely a good thing.

I speak from experience on that last point, I have a borderline pathological hatred of sudden changes of plan and really relate to how you're feeling on the subject.

To decide, maybe ask your girlfriend what her mom is like on long car journeys, if they've been on road-trips before, does mom sing? is the conversation stimulating? Is she awful to travel with? Motionsickness?

If she'll be a good travel companion then your journey might be a lot of fun.

On the other hand, That "we're on our way" feeling when you set off on your own together really detaches you from normal life a lot and travelling with Momfriend might well disrupt that, but it's a fleeting feeling, it'll be gone within half an hour or so. So the question is, Is that feeling really worthwhile?