First of all....I am so sorry that you are going through this. No one knows what they would do for sure unless they have walked in your shoes.
Someone posted earlier that God doesn't give us more than you can handle and that this pregnancy was meant to be since your husband had a vasectomy....I totally agree, but I also believe everything happens for a reason....even though we may not understand why at the time. Maybe God allowed you to get pregnant so that you would catch it earlier since you would be visiting your Doctor on a monthly bases....otherwise, you would have possibly waited until your 6 month or yearly checkup and maybe you wouldn't even have these options.
I know this is definitely something that you and your husband will research every possible options before making any decision. But if it were me....I would choose my children that are already here and the commitment you have to them. You are not being selfish to choose your life and your children you already have. Who would take care of your oldest daughter that is disabled if you were not around? I know your husband would...but it's not the same. I think your first priorities are the children that are already here and yourself.
I will keep you and your family in my prayers. I know whatever decision you make will be the best for you and your family. Everyone has their own opinion...but this is your life and your family and only you and your husbands can make this decision. Either way you have a wonderful support group here and no one here would or should judge you!!!
(((LOTS AND LOTS OF HUGS!!!)))
Tina