Jyefamily,
My wife had TBI, as well as chemo, before her transplant and we were warned that it can destroy fertility. But we already had our family.
I'm concerned if you haven't had an appointment with a fertility specialist. You need to have all of the options laid out for you, so you know your choices, and a specialist in transplant-related fertility is the best person to work with. I know that Johns Hopkins has the expertise (for example see
this article about fertility and radiation).
At your age and with a sibling match, may I ask why you are having a mini-transplant rather than a full transplant? And how do they define "mini" if they are planning total body irradiation?