so it's got to that time in the pregnancy where we want to tell everyone the good news ..but I have a huge concern with telling my SO's sister ..she has had 3 miscarriages over the last couple of years, she has no trouble getting pregnant, but unfortunately cannot carry the baby and she lost her last baby at 18 weeks

this was only 5 weeks ago so it is still very raw and upsetting to everybody.
I am dreading telling her as I don't want to upset her more, or make her feel like we are rubbing it in her face.
We want to tell everybody, but we don't want her to be the last to know, she will know first, besides his parents.
How would you do it? Any advice welcome, Thankyou
Re: Telling sister in law..
TTC#1 May 2009- July 2010 on our own with no luck
Started with RE in August 2010, dx with unexplained IF and then finally our 3rd IUI cycle using Follistim and Trigger resulted in our wonderful little man. Born 12/2/11
TTC#2 Never really prevented, but were careful early on as Dr. reccomended
Surprise BFP 12/16/13, started progesterone immediately as first numbers came back low, but betas were good. Progesterone wasnt enough. Natural MC 12/24/13.
Back with RE as of January 2014...
5/27/14- Chemical Pregnancy
April 2015 IVF#1
5/13/15- BFP, please stick LO!
Oh and I'm a major Harry Potter Nerd
I would have your MIL tell her so she can be there for her daughter. You may feel like a coward for not telling her yourself, but she probably wouldn't be comfy talking to you after hearing this news. It may be better for her to hear it from someone she can cry to, not have to put on a brave face for.
Whatever you choose, it's hard. And it may feel like it's going to ruin your pregnancy time and your relationship with her. I love my SIL dearly, miss her everyday and wish this didn't keep us apart. Just be prepared for this and try to keep moving forward and enjoy your pregnancy. It's not easy, but the only thing you can do right now is be happy and healthy for your new LO. That, as heartbreaking as it is for your SIL, is what is most important right now.
It is really impossible for those who have not experienced loss, or IF to understand.
I would never want to be told about a friend of family members pregnancy in person when I was in the middle of multiple losses. An email or text is absolutely the best way to convey this message, especially with it being only a month since her loss.
Don't make her hold in tears or put on a happy face. Let her grieve in private, she will bring up the pregnancy when she is able to to.
OP Congratulations on your pregnancy, I think it is great you are able to think of your SIL feelings when announcing.