In my opinion it needed two intruders to control the situation properly or the entire attempt could have ended in farce. I have a feeling that one intruder kept Sheila at bay while Nevill was being assaulted downstairs. I don't believe that Sheila ever knew the twins had been killed or she would have been impossible to control. Shona is right about the gouge marks on her as well.
I don´t agree about the gouge marks. To me they look like streams of blood coming from Sheila taking her arm up to the first wound. I have seen this happen. My husband had a sudden, fierce nosebleed. He took his arm up, placed the back of his hand on the nose and leaned forward. To my amazement, I saw the almost exact patterns of blood run down his lower arm as those seen on the photos of Sheila - I mean, it looked so similar, that my jaw dropped. The blood dried very, very quickly making the same pooling at the end of the streams of blood.
I CAN´T see that they are gouge marks, and they look nothing like the marks on Ralph´s arm, which are gouge marks. Made by what I don´t know; I don´t think fingernails, but I don´t really know, of course.