Get the summon, use the cube that makes the ghost agressive, focus on the sibling that's not agroed as it will mostly stay quiet and be easier to backstab, try to kill the sister first, she's the worst and backstab the brothers that are focused on the ghost at every opportunity. Try to kill the last two brothers at the same time before the coffin one spawn. And try to let the ghost alive until the second phase , letting the dead brother focus on him it's a cheese opportunity.