Hi, C.. Well, I would tell different people in different ways.
First, your children. I think perhaps you could have a quiet, special dinner for just your immediate family. Let your daughters be the guests of honor. Maybe over dessert, gently let them know they will be big sisters now, and give them a chance to say how they feel about it. If the two girls you already have aren't close, maybe you could take them each out on a sort of "daughter date" and tell them individually, and let them tell you how they each feel about it.
As for the in-laws, since there is so much tension and conflicting emotions about your SIL's fertility issues, I would advise not putting yourself in a position to have them gang up on you. I might make a special announcement card for the occasion, mail it out, give them each a chance to deal with it, and then speak to them about it. Else, if you're really brave, and if you can trust them to not make a scene, maybe have a dinner party AFTER you have told your daughters about it (don't subject them to everyone else's conflicting emotions while they are first learning the news, especially the teenager!), and make the announcement.
There isn't going to be any way you can control how your SIL and your MIL react to the news. Eventually, your MIL will come to her senses and want to interact with her newest grandchild, but your SIL may have issues that you can't help her with. I don't say that you shouldn't have compassion on her, but she may feel jealous and upset no matter how you break the news to her. My only advice is to tell her in the most loving way possible without rubbing her nose in her issues.
Well, I hope this is helpful. Good luck on uniting the family around the new baby.
Peace,
Syl