Firstly congratulations on your pregnancy and you really do deserve to be happy & announce it when you'd like, however I think it really depends on your relationship with your SIL & how you should approach the topic, while still being sensitive to her feelings.
My story quickly-We hadn't seen my SIL for months, she just drops in & out of our lives when it suits.. Anyways last year we lost our twins at 14 weeks & the only contact was a message to DH (once he had told her what had happened) saying 'sorry to hear'... I think it was about 4 days later, she decides to call DH to announce she is 5 weeks pregnant! I found this very insensitive as she couldn't pick up the phone or pop by to say sorry, yet she magically found his number to share her exciting news 4 days later!
In this situation I would have loved for her to wait a couple of weeks before telling us (as she didn't announce it to other family members until 13 weeks) or at least seem genuine with her 'sorry' message! I was still bleeding for goodness sake
If you're close with SIL, I would pull her & your brother aside just before you share the news publicly.. Who knows, maybe if you wait a few weeks she could be announcing her pregnancy as well & that will be easier on everyone
Best of luck to both you & SIL xx