Bella, First of all congratulations on the new baby.
Secondly can I ask if you are new here? It's just that a lot of MNers will tear you to shreds for not using paragraphs and sentences or text speak, so maybe try and sort that out next time 
Lastly after reading your post whilst I do sympathise with you because 3 children under 5 is difficult especially when one of them is a newborn but your SIL obviously needs help and sees you as an experienced mum so you are the first person she will call if she is worried etc, try and take it as a compliment that she thinks so highly of you.
Explain to her again that you are exhausted and remind her that she will understand in 7 months time, thus acknowledging her pregnancy.
I remember a friend of mine was like this when my DS was a year old, after several IVF attempts and miscarriages she finaly managed to concieve but was a nervous wreck, in the end I had to tell her that whilst I was happy to speak to her when I had the chance, as she would learn kids are not so easy to just work around all the time, that small comment made her stop the constant calls and texts and just sometimes or she would text asking me to phone her when I had the chance. It turns out she had so many people telling her 'to wait and see' or 'don't get too excited' that she had seen it as more of a medical issue than a pregnancy so she panicked a lot more IYSWIM.
You need to be firm and supportive, let her know you will be there for her when you can fit time in. A text or 3 is surely not that much effort when you are breastfeeding.
As for comments like 'im a believer 'if its for you it wont go by you'' or 'yet another miscarriage' they are in no way helpful or supportive, infact it is downright nasty and dismissive of what she has been through and the babies that she has lost.