How to write an invitation letter for us visa?

My sister would like to invite both my brother & my sister (who are in Sweden now ) to come to the usa for a short stay...2-3 months would be ok, we haven't seen them for 12 years. We as a family will take care of all their expenses. I would like to know how to write an invitation letter for the American Emby in Sweden. What should the letter include ..what do we need to attach along with the letter. Thank you & God bless you All. Amen