Yikes! So I'm posting from the perspective of someone who couldn't have children and ended up having a hysterectomy fairly young.
What jumps out at me is that your sister is carrying a HUGE amount of anger and rage.... she is consumed with it and is turning it on anyone around her. Much of it is driven by overwhelming grief and sadness about her miscarriages and that she can't have a child, but you say things which lead me to believe she had anger/resentment issues before.
You say she has been having counselling - which is fine but the thing with counselling is that if you don't talk about what the ACTUAL real issues are, and approach the process with something of an open mind, then it's about as useful as a chocolate teapot. You just spend hours and a lot of money skirting around the issue and never get anywhere unless you address what the actual problems are. The trouble is that you can't force someone to do that. You can point it out to them, you can suggest perhaps seeing a different therapist, you can offer advice, but unless that person wants to resolve the issue, they will never really seek to address it. A counsellor can really only help you with what you tell them. They are therapists - not mind-readers. Also some people go to counselling but refuse to accept anything suggested to them by the counsellor. I had a friend like that - her counsellor was being honest with her and was making very good points but my friend dismissed all of it so it was a pointless exercise in the end.
Sadly, some people seem to enjoy misery - they have become so consumed with their anger that it's become a part of them and it can be easier to them to carry on with that than face up to the cause and deal with it which I suspect is something your sister is terrified of, because it means accepting that her fertility issues are no one's fault, she can't blame anyone for her miscarriages, she can't do anything about it - it just is - and that is, to be fair, a difficult thing to accept. However, she cannot expect everyone to put up with her behaviour forever and she has a responsibility for herself and her own happiness. No one else is ever really responsible for your happiness.
The things your sister has said to you carry so much venom - it's like she's spilling over with so much anger, bitterness and jealousy, that it's almost projectile the way she spits it out, and you absolutely do not have to put up with that - it's vile. She seriously needs EFFECTIVE counselling to address her issues - but as I say - that comes down to her at the end of the day. If she doesn't WANT to find a way to move on (I don't mean forget or dismiss her miscarriages - just find a way to process the grief, to live and move forward), then there's not a lot anyone can do.
I do think it's a bit of an unfair to comment about her being fertile just can't carry a child to term; that's not her fault - nor is it anyone else's - and it's no less difficult than not being able to conceive at all. In fact in some ways it's worse because you can get pregnant and have the hope - and then have to go through the agony of losing the child. I was told before my hysterectomy that my uterus was such a mess that even if I had been able to conceive there is not a chance in hell I'd have ever carried to term - I'd have miscarried every time. It's hard to hear that and I have a great deal of sympathy for her situation and what she has been through - that must've been awful.
However, in order to function and have any kind of life, there comes a time when one must accept one's situation and find a way to deal with it, otherwise you just spend your whole life bitter, angry, isolated, and consumed with what you cannot have. I had a bad time right after my hysterectomy - I found it so hard to be around people with babies and I felt like I was grieving for the child I'd never have. I wasn't a nice person to be around - I freely admit that. I had counselling - but I wanted it and from day one I was very open about why I was feeling like I was. My counsellor helped me process it, own it, deal with it and find a way through it. Now I'm completely okay with the situation, I'm actually genuinely okay with not having children and I have found lots of other things in life that I enjoy, which actually, i wouldn't have time for if I had kids. I get to enjoy spending time with other people's children and it's wonderful. I have reached a place of peace with it all.
Your sister seems to go on about how happy she is without a child etc... sounds to me like she's trying to convince herself - but is convincing no one else. She's hoping if she says it enough it will be true. But she is far from being in that place, in my opinion. The bottom line is your sister is struggling massively with a lot of issues - many to do with her miscarriages - but I suspect a lot of other issues too. Unless she takes responsibility and addresses those issues openly and honestly with a professional, she's unlikely to get anywhere.
So how do you deal with it? Well, I've been around people who love their own misery and sadly there comes a point with some people where you have to accept that you've done all you can and it's now up to them. If they will not do anything to help themselves, then all you can do is limit your exposure to the negativity because it really does become toxic. Maybe the time has come to say to your sister "look, I understand you have been through hell with your miscarriages, and you clearly still have a lot of grief and anger about it, which is fine, but what ISN'T okay is to use myself, or anyone else, as your punching bag to get rid of your anger. You need to address the real issues honestly with your counsellor, and if that counsellor isn't helping you need to find another one who is a better fit". Sometimes one has to be a little bit blunt, point out that she has a responsibilty to deal with her issues and no one can do it for her... nor is it anyone else's fault what has happened to her. You don't have to be nasty - but be straight with her. Perhaps you, her husband and anyone else in your close circle could have the conversation with her together - tell her you are all concerned and care about her but none of you are prepared to put up with her abuse anymore. Then back off - get space, go low or no contact for a while. She may kick off but with time to think she may realise you are all correct. It is then up to her - if she deals with it, then praise her for it and encourage her to keep going in a positive direction. If she doesn't - then you may have to accept that she is someone you cannot have around you very much - if at all.
Good luck with your pregnancy - remember you can't control what other people say and do - but you do have power over your reaction to it. :-)