Need advice on how to share our news with family...?

My DH's brother and his wife have been ttc their second child for over a year without any luck. My DH and I started trying for #3 only last month, and this morning I got my 2nd BFP (1st was very faint, wanted to be sure). We had been unsure for a while about whether we wanted more, and our family new that, we didn't tell them we had decided to try. I am really close with my sister-in-law, but didn't know how to approach the subject. I don't want to bring any sadness to them, but we are very excited about the news. Any advice on how/when to tell them would be very appreciated. Thank you so much for reading my question, and baby dust to all.