It doesn't matter if they had motive or not. The phone call makes the killer either Jeremy or Sheila. You can waste you time looking for motives under stones if you like - but the call makes it one or the other.
Maybe lookout watched too much Tube and subscribes to the following:
A) the relatives did it for the money
B) They forced Nevill to make the call to Jeremy to sucker Jeremy there so they could kill him too (for some reason not fearing he would summon police and thus get caught at the scene) and either frame him for it or Sheila and for some reason actually Nevill complied and summoned Jeremy
C) Jeremy didn't rush over as hoped but instead for some reason called Julie then finally decided to look up police numbers
D) In the meantime they got scared because the amount of time that passed and feared being caught at the scene so framed it on Sheila and split
If one is totally under Jeremy's spell they ignore how unlikely it is that someone would risk Jeremy alerting police instead of going over alone to be killed and how unlikely it is that Nevill would comply. Also how unlikely it is that June would be in bed during all of this so that Nevill could be marched upstairs and shot with June before running back to the kitchen.