No-one can possibly give a sensible comment about this without knowing more about your family situation. From what you've written, I think it's wholly understandable for anyone who has had post-natal depression to be worried about it happening again to then try to PREVENT it happening again. which would logically to be not to try for more children. That's all there is to it. Nothing to agonise over. It isn't that your mum didn't want more children per se, or didn't love them, it was because she was terrified of her illness coming back if she got pregnant again.
As someone who was on the verge of PND myself and found the first 2 years with DS2 really hard, I do not want more children, because what if I experienced the same again? That's not at all to say that I did fall pregnant by accident that the baby would not be loved. Not at all! Ds2 is now the absolute light of my life and brings so much joy to everyone and I'm assuming that that is exactly how your parents felt about you once you came along. It is nothing personal, it wasn't to do with YOU that they hadn't planned more children. As long as you felt loved when you were here then I'm not sure what the problem is?
How many times do we hear of an older mum's pregnancy and they laugh and say "oh yes, it was a bit of a surprise, let's say!" And the child is loved as much as any of the others.
It all boils down to the question: did you feel loved by your parents? If the answer is yes, then you are overthinking the PND comment and should let it go.
Incidentally does your brother feel as hurt as you? After all, he could feel worse off than you if he (wrongly) believes that he was the "cause" of the PND in your mother.