To be absolutely furious with my father?

you need to take him aside and gently or not so gently tell him this won't be the case and state what you have said here
take it as good opportunity he disclosed this view to put him straight

A baby in their 50's who the hell things this was ever a good idea medically?
So selfish to the future child on so many levels, hit 20's and possibly a carer for their aged parents when they should be living their young lives as they should.

I'd be really annoyed too that he hadn't had that conversation with me .. let's hope this never happens. But I think I'd look after them if they had no parents

@Namerchangee that is not what she is saying and that is not what this is about. It's about her father making very irresponsible life choices and assuming OP will be his back up plan. If I were OP, I'd be miffed too. There will be other relatives out there too, why should OP step up? It will be a decision made at the time, depending on everyone's circumstances.

As my DM - who had my DB and me at 21 and 24 - once remarked about women like this - "She needs her bumps feeling". Having had my third naturally at two weeks short of 39, I have to agree. Madness.

That's the sort of conversation a couple having kids in their 50s should be having before getting pregnant if they are expecting OP to look after her half siblings after their death.

Writing as someone who had an older dad and older half-siblings, this is quite upsetting. These children are your half-siblings, not just some kids! Your Dad isn’t exactly old at 50 though - my DF passed away well into his 80s, so in reality you’re unlikely to have to consider yourself as a carer for them - but the fact you would consider seeing your siblings go into care says a lot about you OP. It’s not good.

It's so unlikely to happen, I wouldn't be so furious about it. Even if it does happen, there is zero legal responsibility on you. Whether or not you take them in, will be entirely your choice at the time.

Given they are having a baby in their 50s, they probably have other issues and I assume this is just tipping point for you. Let it go. This is not the hill to die on as it's so unlikely to ever materialise.

Just remembered to ALWAYS say NO to every single babysitting request that will come your way. I think it's more likely they will ask for more and more help as they get more tired, but you need to say no from the beginning.

The nerve of the man is staggering. And how many children does he have now?

They're selfish imo. There's a reason why 50year old + women can't conceive naturally.

why would a 50+ year old woman have IVF to have a third child!!! she clearly needs her head looked at!! I look after full time a 14 year old and a 16 year old at 70 so I know just how knackered she is going to be!! going to be well into their 70's before kids leave uni etc!! that is if they are still alive at that point! it is no fun I can tell you!!

I hope the kids were’t around to hear, I still remember being the subject of a very similar conversation 30+ years later 😅

He definitely should have had a conversation with you about it before. At least he knows it’s not an option now so he can start thinking about it seriously.

I don't think it's harsh either. It is the harsh reality they have chosen for their DC. They have a higher than average chance of not getting their DC to adulthood due to their age. They should have thought about the 'what ifs' in more detail before having them.

I doubt OP has a fulfilling sibling relationship with these much younger children.
So she would never benefit from them.

She has 1 life. Its now hers to live.
The parents were selfish in creating these children. This is very much their problem.

Not every problem in the world is ours to fix. Sometimes you've just got to let it go.

Indeed if my mother suddenly plopped another kid out I would certainly not be taking it in.

I stopped at three for a reason and had them at an certain age for a reason. Not to end up raising a sibling because my elderly mother suddenly wanted another go at parenthood.

And yes I would take in my child’s child. That would be an unexpected death but an old person dying is pretty expected.

I’m childfree by choice but if I had siblings who were orphaned I would absolutely take them in and raise them. How could any normal person let their siblings go into care?

Why? They aren't OPs responsibility. She hasn't made these children.
She has finished having children

The parents of these children need to make the correct provision for them.

If they are in their 50s, chances are that at least one of them will survive long enough for the baby to reach adulthood, so it's all hypothetical now and I wouldn't worry too much about it. Even if it does happen, you can always refuse to take them in and your dad won't be able to argue back as he'll be dead. Not your kids, not your problem.

Wow, that’s harsh.

I'd be the bluntest/ cruelest you could be to ensure there is no grey area.

Dad and Step mum. Your children will go into care before they come to me. My child rearing days are over. For good.
I am not even a consideration in any capacity.

What if you died first and nobody wanted your children? How would you feel about that? These children are your siblings.

When you have kids it seems sensible to make plans for what would happen if something awful happened to you, the parents: appoint a guardian. This doesn't mean that person would have to take them in, but would be responsible for making decisions on their behalf.
In your situation, the DW would have to become incapable of looking after them, as well as your dad. It's very unlikely to happen. He should've talked to you before mentioning it, but sounds as if he was put on the spot.

Does she have grown up kids?

YANBU at all.
Hopefully you'll never end up in that position, but just to assume with no prior discussion is madness. I'd be furious. I had my children in my early 20s and now have a 17 and 19 year old. I have absolutely no desire to be a mum again (or time or space) and I'd be furious he'd just assumed. It's a BIG assumption.