Help...I dont know how to process any of it

Im 28 weeks pregnant and on the 10th I find out that my baby has a Rare condition of an extra Chromazone 13 and 18. (It's not Down syndrome) and the doctor's think its best if i Terminate my pregnancy. Normally the baby's with this Either dont make it to full turm or they live for about 10 days. I have 2 amazing boys as it is, but I don't know what to do do I keep the pregnancy going and bond with what ever time I have with her and let my boys see her. Then have to try to explain where there sister went or do I just give up and abort her.

My baby's are 4 and 1 what would I tell my eldest son what happened. He's exited to meet his sister and its braking my heart.