If Sheila had been as spattered/covered in the amount of blood that June was,there'd have been little doubt of who the shooter would have been.
Let's not forget that June ruined Sheila's life to such an extent and Colin had been anxious about his boys being indoctrinated the same as Sheila had been,so being that many guns were to hand,whether able to use them or not,they became the obvious weapon to use in which to put an end to all the suffering.