@hollister,
Who are your boys with when they are not with you? My guess is their mom. So in that situation - it is pretty much you cannot be with them - not that you do not want to be with them. So it isn't like you are giving up your kids to be her child, or is it?
What would happen if this child of hers was your child and you had a third child - would you feel that you were not giving proper attention to your two boys? No, to me what is the difference then. If you are serious with her, this young boy in a sense will be your boy too.
Happens all the time. My brother is married to a woman who had a daughter with a previous husband. They now have two children together and the older girl from the previous marriage. To him (and the rest of our family) this older girl although not of blood relation is as much a part of our family as his blood daughters. She is my niece same as her half-sisters.