I'm a little confused as to why your brother would allow you to use his TFSA since it just screws him moving forward if he wants to use it. If this goes to zero, he misses out of 31k contributions.

Chances of that are slim obviously but I hope he's not a younger brother that doesn't know what a TFSA is or what it means down the line. Explaining to family members how to use RRSP's and TFSA's is the more responsible thing to do.