Is this reason allowed for mixing indoors tier 2.

[ImNotCutOutForThis]

We're staying with my df for the weekend. He's a single parent.
My dB has just called. They've had to leave their house as there's a gas leak next door. Only place he can stay is there.
Gas board hope to make it safe tonight but they have 5 young kids at home so ideally need to stay the night rather than move the kids at silly o clock if they get go ahead to go back.
My df is worried he'll get in trouble

Probably technically against the rules but if there is space at your df for everyone, the circumstances are pretty extraordinary so I would just go ahead. I doubt you would get reported (I think you would be very unlucky if you were)

It’s fine. Go sleep in one room use a separate bathroom

OK. I would happily go home but the dcs are all asleep.

Far safer to stay over in an emergency than stay put with the risk of a gas explosion.

Making sure rooms are ventilated regularly will go a long way to reducing any risk.

It's fine. One of the exemptions in the covid rules is "to provide emergency assistance".