No one said that it would be easy, but again, my point was that children should have a relationship with their extended family and although it may be hard for you to let them go, it's a great experience for your girls. I do understand your heartbreak though as I have already been away from my son on several occasions, some voluntary, some involuntary.
That being said, I have a 2 year old son and an 8 year old step-daughter who lives 1200 miles away, so we don't see her very often but when she's here, she's here for long stretches. However, we have lots of pictures of her in the house and he definitely knows who is 'sissy' is. Does his heart break when she goes back to her mom - no. Does he ask where she is - yes. Same thing happens when our family comes to visit. He notices they are gone, but isn't old enough to have a broken heart or anything. He has a photo album of all his out-of-state extended family and loves to look at it often.
So, depending on his age, just keep your daughters in every day conversation (which you would do anyway), give him a couple of pictures of them (if he doesn't have them already), and depending on his age, have him talk to them on the phone. My son 'talks' to his grandparens on the phone and although it's 99% them and 1% him talking, he still smiles the whole way through. We also have a webcam that we use with our extended family. Probably not something you can do this year, but you may consider it next year if your daughters make the same trip.
Good luck!!