How should I break the news to my sis in law and close cousin who've been TTC for months?
Honestly I would tell them just as you would anyone else. I went through a similar situation with my bff the last time I was pg. She had been trying for over 2 years and I didn't want to make her feel bad. I ended up telling her right away and she handled it well. Her cousin (and good friend) got pg around the same time as me but did not tell her until she was 9 months and my bff was very hurt that her cousin felt she had to hide it from her. She said that yes, she was sad that she herself was not yet pg but she still felt happy for her cousin. The whole situation took a toll on their friendship.
Whatever you decide to do I hope it goes well :)
I wouldn't wait unless you're planning on waiting to tell others.
As for the best way to do I HIGHLY recommend by phone. E-mail is just so impersonal but in person can be pretty high pressure.
It took us 6 years to conceive ds & almost a year this time (this time was shorter 'cause we knew what the problem was to start off) & it was almost always upsetting to find out others were pregnant. Not because I was not happy for them but because I felt so bad for our own situation (which made me feel guilty for not being more happy for them). When I was told on the phone it was SO much easier 'cause I could be happy for them & then politely excuse myself & cry alone. In person meant maintaining a happy face that much longer.