Sheila loved her father, and he was one of the few people who could calm her down when she was having one of her episodes.
I believe that Sheila shot June first, because she, and the twins did not like her because of her religious beliefs. " I hate this place". Colin was not happy with June either. To be remembered is June had to go to a mental institution because of a breakdown, probably brought on by the fact she could not bear children. For a women this must be devastating.
All the pictures of June shows a happy woman, but so do the pictures of Sheila, Yet both were unhappy and suffering from a mental illness.