Yellowox you are being unreasonable.
I don’t know how many rounds of IVF your SIL has had, I had lots, it is very stressful, it is very expensive (unless you are on the NHS) and it is both physically and emotionally draining for most people.
You are advised not to do anything strenuous, not to swim in a pool etc.... and maybe because it has been so difficult to get pregnant you do feel more protective of your baby and pregnancy.
It can feel you have beaten the odds to get pregnant so you feel your pregnancy is more precious! Of course it is precious to you, as each pregnancy is to the woman who is expecting a baby/babies.
Maybe her sickness is terrible, who knows what feels worse for one person or for another.
I only had one baby and through IUI not IVF and I almost lost her at 15 weeks, and ended up almost going into premature labour. Well, the hospital thought I had. The whole thing was very stressful. And you know if the pregnancy fails you will almost certainly not pregnant again easily, so you know if it does not work out you are facing more rounds of IVF on top of the terrible sadness of the loss of a baby, which all who lose babies feel. I think fertility treatment leads one to feel both emotionally involved but also not, by that I mean some will have many rounds of IVF, being expecting to be expectant many times and might lose many pregnancies before they really start. All this makes a very stressful brew which it seems to me (talking to friends who just 'got pregnant naturally') is not necessarily present in a pregnancy generally.
... also keeps going on about how her babies should be very intelligent because she’s been taking extra folic acid & supplements for a year. Maybe they will be, maybe they won't - does it matter?
Re I'm trying to bond with her over the pregnancy but she's acting like the only person who has ever been pregnant. Well, this is the first time she has been pregnant, presumably, so cut her some slack. She obviously doesn't feel she needs your advice at the moment - she may one day! So why not bond over something else, a shared love of ice cream, a gentle walk in the park or shopping trip.
Re She also said she won't be going to term (which is common with twins anyway) because her babies are more precious, she said this in front of our cousin who had a very late loss (7 months). It's difficult to know whether this statement was made on purpose to be cruel to your cousin, which I find hard to believe, or was just a bit thoughtless. If someone has lost a baby it is very hard to say anything about babies which might not be considered a bit hurtful or inconsiderate but likewise if someone is pregnant (especially for the first time, especially after IVF) then it is hard to not talk about babies. If she says anything else thoughtless it may be worth reminding her in private that your cousin lost a baby. If it was a long time ago she may have forgotten. Don't shout at me for saying I may forget something like that, people do forget stuff all the time. It's sometimes hard to remember who has experienced what. If it was quite recent then maybe that is less of an excuse!
Re I know IVF is very a hard emotional thing to go through but I can't seem to stop wanting to bang my head against a wall when I see her. I think you need to chill, let her enjoy her pregnancy, she may have felt during IVF that she would never see this day, maybe she wants to bang her head against a wall that other people do not get how bloody amazing this is for her!
To save everyone's heads and everyone's walls just allow her to be a bit precious, be ready to enjoy your new nieces/nephews when they are arrive and don't take it personally, I don't think it is personal to you, it is personal to her.
Good luck.