It's not unusual ( breaking out into song here 
) for the condition to manifest itself years before without realising what's going wrong with themselves. The one that I know had put up with her " feelings " for all of 17 years ! After much researching it had been post-natal depression which the woman used to mask----as you do---with substance abuse to "feel better". 25 years ago it wasn't as recognised as it is now, but that's how it all began.
With Sheila it was even longer than that. More often than not it's a hormonal imbalance that creates havoc as there didn't appear to be anything wrong before the twins were born, or if there had been it would/ should have been picked up during her stay in the maternity hospital.
It seems that, after 17yrs, the poor lady was diagnosed. Can depression be described as being "post natal" after all that time, given that there can be hormone problems surrounding the menstrual cycle, too. Suffering both at the same time would be a bit of an anomaly, I imagine.
We really can't know -without knowledge of her gynaecologic history- if Sheila had suffered from severe/mild hormonal imbalance prior to the twin's birth. It's difficult to know if her depression was clinical or a result of life circumstances, OR whether either became exacerbated by her schizophrenia.
