He wouldn’t really know. Based on Shen Feng actions he made sure SYX had the basics of care, Shen Ling mentioned before that he never noticed her and it wouldn’t be odd if he forgot her existence. It happens in large families. Plus it was the twins that made it so she rarely had food and while the maids and servants helped with that we also saw that the twins made sure there was no witnesses to most of their acts in SYX room. None of them cared enough to investigate as it’s just not done in this time period (honestly given the period it’s a miracle she wasn’t killed or abandoned the moment they realized her intelligence in such a day and age whether they were wealthy or not there’s no one that understands the correct care for those with special needs.) When her intelligent returned, she never said anything about how she was treated by the servants or her cousins and influential households tend to be extremely hands off regarding family members that cannot benefit or threaten the family. It may seem cold but it’s normal in their day and age, you can’t judge based on the logic and mentality of the 21st century.