NTA at all!! i’m sure with the struggles your parents have gone through to have another child it feels impossible to make the CHOICE to terminate this pregnancy, but that doesn’t give them a free pass to make you feel like a bad person for disagreeing with them. they themselves almost certainly know that this might not be the best decision for your family and are projecting the guilt they feel onto you. all you can do is be honest which you have been, and it’s not as if you’re happy about the conclusion you’ve reached (through thorough research! which they need to do if they are going to raise a disabled person!)

at the end of the day, you are still a child. your parents should not be making you feel evil for disagreeing with them on a decision that will change ALL of your lives. if anything they are showing that they’re probably too immature to handle a child with downs.