All very fair points; I was thinking (but didn't specify) of the step-brother/sister relationship as mentioned in other comments. And just to be clear, I'm not

All very fair points; I was thinking (but didn't specify) of the step-brother/sister relationship as mentioned in other comments. And just to be clear, I'm not an especial advocate here! I'm just interested as always in establishing clear logical rules that can then be applied fairly.)