AIBU to expect my sister to accept my fourth pregnancy when she is child free not by choice ?

You’re not unreasonable to have 4 kids but you are unreasonable in your expectations of her. I’m always amazed at how people who conceive easily don’t understand how painful it is for those who don’t, or who have otherwise “missed the boat” in the sad way your sil has. And I say this as someone who has got a child but who was in both of those categories at one point while watching family members pop them out. I could barely speak to them during that time (and had to keep my distance to avoid lashing out), and I still feel upset remembering it. Just give her space and let it be.

Well no. It doesn’t work like that.

I don’t get why she’s mad at me

That is spectacularly obtuse.

In an ideal world, she'd be happy for you, but this is not an ideal world, and she's in pain - a pain that can only be fixed by time, and those around her trying to fucking understand, instead of judging her.

Could you try to have some imaginative sympathy and just try, just for a couple of minutes out of your own head, to imagine what it must be like to see the man whom you loved & to whom you devoted your fertile years, keep on saying "No" to having DC, then when he's divorced from you, he goes off & has DC with someone else?

Just try to have some empathy, or imagination.

I think that’s the case? No wonder she made a comment about his endless shagging.

It must have been awful when your sister‘s ex went on to have children with someone else. I can’t imagine how painful that would be. I would guess that she thinks six kids for your OH is excessive-he will be 60 when your baby starts high school which is a bit strange.

So your husband will actually have 6 children with two different mothers 🤯. Thank goodness he’s stopped now!

I absolutely disagree with this as someone who wasn’t able to have children. I never made my infertility someone else’s fault. Longing? Yes! Grief? No. That’s my own experience of course but I’m tired of infertile women getting a pass on being so rude.

Thing is if she is 44 now and has a partner now who has children. Take a guess she’s been with him 3-4years?

There will have been a gap likely in her 30’s single where she could have tried donor sperm ivf / Ics? And she decided not to try that.

Her and her current partner if they have been together 3-4yesrs again she could currently even do ivf with his sperm if he was up for it.

She made choices even after leaving the ex which has stopped her trying for children even pre this partner.

You say she’s not infertile so she just never tried and now she’s mad at everyone who has children.

There’s probably nothing else you can do.

At the same time - there is no “should” about feelings. Just as she can’t demand that you refrain from having a fourth baby (and like you say, that ship has sailed), or from being happy about it, you can’t demand that she be happy to have nieces and nephews.

Maybe she needs some distance, and you need to allow her to have it, while at the same time leaving things open in the hope that she will want to reconnect in due course. And probably in time she will come to love her new niece/nephew as well as the ones who are already here.

I’m interested in your mum’s perspective - she said you should accept that your sister might not want much contact for a while, and I think she could well be right. Did she make any other suggestions as to how you can improve the situation? Remember that as much as you love each of your three children plus the one on the way - your mum loves each of you that same way.

Will this be your husbands 6th child?

Again, this is very easy for you to say. I think you sound quite judgmental & unempathetic towards your sister.

Just be gracious. You've been lucky; she was very unlucky. Don't assume you are better, wider, or more deserving than she is; it's mostly luck. Lucky that you found a good partner; lucky that you are fertile. You're not superior in your judgement, nor is she inferior in hers.

Your sister made some very hurtful comments and that was wrong of her but it sounds like she is dealing with grief and one that time may not help. Of course you are not wrong to want to add another loved child to your family but she may not ever be in a position to be happy for you having a life that has been unobtainable for her.

Also the comments saying it is effectively her own fault being child free due to staying with the wrong man are really unnecessary and hurtful, and if you have made these comments to her I could see why she is upset. Sometimes staying with a partner that everyone else can see is wrong for you is not that easy to see from the inside. She may have been looking at the sunk cost fallacy and thought it was more likely he changed his mind than her being able to find another partner who would give her what she wanted. It is so, so unfair to blame her and say it's her own fault she has no kids as you do not know what went on in her relationship and she probably blames herself enough as it is.

Absolutely not the Op's fault.

But if someone wants space, you give them space.

I am also child free - and not by choice. Yes sometimes I do feel sad about it. However I would never be unkind to someone who is pregnant. It's not their fault I didn't have children.
Whilst I appreciate she is hurting, it's not the OPs fault.

That’s incredibly tone deaf and this attitude will contribute to her reaction.

I'm childless - not through choice. My feelings are not someone else's problem.

Omg. You really don't have a clue

You are both entitled to your own feelings, you are entitled to be happy, she is entitled to be upset. Not expecting her to be happy for you and showing her some gentle consideration would be generous on your part.
The person being unreasonable, I feel, is your mother. She shouldn’t be telling you how your sister may or may not feel or act once the baby is born. She doesn’t know and it’s actually nothing to do with her. She should be supporting both of you if anything. If she can’t do that for you as well as for your sister then put some distance between yourself and both of them and concentrate on your own little family and your own happiness.
Just because your sister can’t have what you have (and, yes, that is painful for her) doesn’t mean you’re not entitled to it.

Are there other situations that she states/implies are someone else's fault, beyond her control, unfair etc.?

Why is it hideous? The sister made a choice, and it didn’t pan out. That’s a simple fact.

Four children really is a lot these days. At least OPs husband has, belatedly, done the responsible thing.

I spent years struggling with fertility and miscarriage after miscarriage. It consumed every fibre of my being. What I will say is I wasn’t really happy to have nieces and nephews, I didn’t really care tbh. So I think YABU for that comment because even if she was overjoyed and obsessed with your children it isn’t equivalent to having your own child.

I think your sister is BU for her comment to you though. You can quietly keep your distance from someone without making them feel guilty for their fertility. I pulled back from many relationships and would have never said that, and I only would have mentioned it to that person had I been pushed about my distance.

I found particularly solace in a group of women who also struggled like me and we propped each other up in our most difficult times. They inevitably cut me off when I got pregnant (after several failed rounds of IVF) but they didn’t make any comments like that to me. I have reconnected with a few of them now time has passed. A couple managed to conceive and one recently adopted.