Complex sentence: Margaret had acted as substitute mother for the remaining children, Frederick Jr. and Charles, but in late March 1870 she died from an undetermined stomach ailment, leaving Mary Ann to console the grieving Frederick Sr. Soon her eleventh pregnancy was underway.
Split into simpler sentences: Margaret had acted as substitute mother for the remaining children, Frederick Jr. and Charles. But in late March 1870 Margaret died from an undetermined stomach ailment, leaving Mary Ann to console the grieving Frederick Sr. Soon her eleventh pregnancy was underway.