Teens Might not be too keen in the next addition to the family?

Hello there, Well i have two teenage boys one 13 other 14,there mom isnt in the picture as she betrayed us about 12 years ago but i re-married again to the most amazing person in the world and the kids adore her too.We had one baby together so far and at the time it took them very long to get used to the idea of someone else joining our family.I spent a while trying to get them to see they wouldn't be forgotten and after a long few months of talking they agreed! My daughter is now 2 and my wife is pregnant again she is 4 months gone we were going to tell them last month but there hormone are all over the place i'd say .Since my boys have reached teenaged years there kranky moody and stubborn.How do me and my wife tell them we are having a child for the second time without them going all 'Woah man' ? there not easy to talk to ? and does anyone out there have the same problem?