Hello @geek
Your letter isn’t bad at all, but you said it only concerns accommodation and living expenses, so why are you still talking about tuition fees in the letter?
Anyway, you could mention the exact amount she will cover, and the amount the other person will cover.
It’s better to use the one with 18m2. In my opinion, it’s too risky for the other, unless he renews his receipt in the meantime. A recent bill in his name less than three months old would be ideal.