I would involve them as much as possible in your pregnancy. the more connected they are to the baby now, the more their hearts will turn. after all, that new bundle of JOY will be their half sister / brother. :)
You should talk to them about respect and the fact that the new baby will be their little bro/sis... Once the baby is here I'm sure it will get better plus its a great form of birth control for them lol but those are teenagers for ya!
They'll warm up and love playing with the baby, its probably more of not wanting to share dads attention. You should take them to the ultrasounds they'll think its cool