Letter of invitation?!

First of all I want to thank you on this great forum where I found a lot of things about immigration to Canada and God bless the person who open this forum:-)
Well,I have a question if anybody knows the answer,I will be very grateful
I am here on first extension of my tourist visa staying at my boyfriend who is my sponsor and his brother wants to come to visit us for 2 weeks in September and he need the tourist visa,so he need the letter of invitation from somebody here to come here,of course and we were wondering can my boyfriend send him the letter although he is sponsoring me here?!
By the way-my boyfriend is here on working visa-he sent all the documents for permanent residence and we are waiting for PR
If anybody knows anything,please,write me
Thank you