my boyfriend is telling me i need to extend an invitation to his sister's boyfriend so he can visit
Why would you have to extend an invitation? Why can't your boyfriend do it? I think what is meant by the "letter of invitation" is more a sponsorship....meaning you are vouching that the person is coming into the US under a tourist visa and is financially capable to take care of himself while here, eliminating the likelihood that he's planning to stay and work, thus becoming an illegal immigrant. If you dig deep enough, I know there's info on it in the 2nd website you posted. Good luck!:)