If rumours of incest were true then this provides a motive for murder and that David should for some reason be spared

Likelihood: 
0.001

The Defence went to great lengths to explain that there has to be a motive for the murders and that an incestuous relationship between the Robin and Lanier was sufficient to motivate him to kill the entire family apart from David.  

If you use statistics of the numbers of people who commit incest and who also kill their entire families then the odds we are looking at are near zero and this might be the only recorded case in human history. Despite what we might like to think, incest is relatively common however incest leading to murder is rare.  A figure of 0.001 is probably very generous here.  This means that we are saying that 1 in 1000 people who commit incest also kill their entire family or at least one member for reasons directly linked to their offending or we could be saying that 1 in 1000 murders involve an incest victim being murdered or murdering the perpetrator.  Either way, the link between incest and murder is not strong.

Further to this there is absolutely no explanation in the Defence story as to why David should be spared.