Nevermind waiting for the white sheets, because I splurged and bought a whole roll of white cotton for making toiles. It was £30 for 30 meters so I didn't exactly break the bank. In fact, it isn't that much more than my brothers shirts will end up costing. Each.
I managed to cut out all the pieces of younger brothers test shirt today and if all goes well I'll be able to sew it together tomorrow and then it'll be ready for him to try on over the weekend. :-)