He knew her - we don't, it's SIMPLY a possibility.
We know she depended on her parents for financial support and her father for emotional support. We know that her doctor said she was extremely close to him. We know her doctor said her relationship with her mother was improving and was better than it had been in years.
He would have to expect the possibility (and even likelihood) that if he approached her saying he wanted her to help kill them both that she would tell someone before he had the chance to carry it out.
That would ruin his long term planning. Why would he do that? What aid would she be able to bring to the table in helping?
Engaging her help so that she would make sure they switch beds would not help at all. In any event the evidence establishes they didn't switch beds but rather that both parents were attacked in the master bedroom and that this was the start of the event.
If people want to ponder they can ponder whether Jeremy ran away immediately after the gun was empty to reload or Nevill somehow got passed him and fled. They could wonder whether there was a mini physical fight in the bedroom where both parents went after him and he punched the or the fleeing to the kitchen was immediate. These are variables that we can't be sure of so people can ponder them.
Unless Jeremy decided to talk honestly no one is ever going to get an answer though. The wondering of which of these happened will never be answered no matter how much we ponder it. I accept that. I face the things that we will never know and since it makes no real difference in the grand scheme of things I don't worry about it. People who have so much curiosity that they have to know everything no matter how trivial do care but caring won't make them able to figure out which of the possible variables is the one that happened so it still accomplishes little except driving them crazy that they will never know.