What do you want to address / need to address in the letter?
The application to sponsor a family member is based on a checklist of documents that is required. If there's more information that you wish to provide than what can be included on the forms, you can write a letter of explanation detailing the relevant points; but they should relate to the application. There's no need for a
cover letter to accompany your application. If a letter of explanation is required, then it should be placed in order among the documents according to the checklist to the section that it applies to.
Perhaps if you gave us more information about what you want the letter to say, people can give you advice and tips. Though if you're just looking for an example letter following a Canadian standard then
here's a model (go to 10.26 on the page) following the Canadian Style as used by a government agency.