Is this rent amount ratio fair in a shared household?

[Mumofacutie]

Hello, I am here for some opinions, really.
My auntie and husband live in a 4 bedroom house. They share one bedroom and the 3 other tenants each live in the rest of the rooms as singles. Bills and council tax are not included with the rent and they split that into 5, which is absolutely fair enough. Now my question is if it is fair that the rent amount is split into 5 as well? So let’s say the rent is £2000, so the singles play £400 each and the couple pays £800. The size of the rooms are about the same.
Is it how this works? How would you do it?

Rent split by room plus a little extra. Not double. They only share one room but there are 2 of them using public spaces and bathrooms.

This is assuming no en-suite etc.

I hope it's an official HMO and all safety regs are adhered to.

Thank you! No, no the bathroom is shared as well

What you describe sounds fair to me.