First off congratulations, I know you wanted another LO. I read back on some of your old posts. Just curious, did you and your fiancé get married yet? Maybe you guys can find your own place.
Your parents raised you and have been helping (I am guessing) you with your current LO. So adding another LO to their household is a lot to expect and it is unfair to expect them to be receptive to that idea. I doubt they want to continue to raise children and may want more free time to relax with each other, travel etc. That is just my guess.
Do you still work in the NICU? If so, that is great because you now have a means to pay your rent/mortgage. While you are pregnant save, save, save so you can afford to care for you and your children while you are on maternity leave WITHOUT ADDED STRESS.
You will show them you are taking some responsibility by doing your best to take care of your children. Look into state assistance because depending on where you live daycare vouchers are available, food assistance etc. I am not aware of your finances, you may not need it, but at least you know you have some options.
Your parents may even assist you in finding your own place and maybe you can move close to them. Good luck to you.