This route works differently from the others. There is no file for a consular officer to weigh before the interview, and most documents are never read. A sponsor letter still has a job, but it is a smaller one, and it is worth knowing that before you write it.
It answers one question your relative will be asked at the window: who is paying. The answer has to match the DS-160 and it has to be immediate. A letter that says what you are covering, with your statements behind it, means the answer is specific rather than vague.
It cannot supply ties. 214(b) is about your relative's life at home, not about your resources here, and a wealthy sponsor does not overcome the presumption. In some cases a strong sponsor makes matters worse, because the visit begins to look like a move.
A short letter with your name, status, address, relationship, the dates and what you are covering. Your statements. Nothing longer than one page. Your relative carries it and hands it over only if asked.
On their answers, their DS-160, and their evidence at home. That is what decides the interview. Our software checks all three against each other, and says plainly when a sponsor letter is not the thing that will change the outcome.
You can open the case, pay for it and add your own documents, then send the link to your relative so they can add theirs. One purchase covers the case whoever pays.
Our software reads your documents, checks every ground, tells you exactly what is thin and what would fix it, and writes the cover letter, the sponsor letter and the employer letter from the facts you gave. One re-run included within 30 days.