I am sure your mother understands that money is needed to build a house and not one of you have that amount. Also, you are not their only child and if it's more for your parents everyone could give a hand which isn't the case. We can't do more than our best.
I understand you like to build it in one piece but even if it comes to this you might have to make a compromise. A little house with sun, dry, none flooding floor and a part where you can sit outside already or continuing like this.
I do agree with you it's wise to have put money aside for your health first since without that there's no way you can achieve anything.

I hope you have a good Sunday