How do I go about sending an invitation to my sister-inlaw in Dominican Republic to come visit the US?

You don't. She has to apply on her own merits. And having relatives in the US is a negative - too often a sign of intent to remain, visa violations.

She must prove she has sufficient funds of her own (none of yours) for all typical tourist expenses: round-trip tickets, hotels, restaurant meals, sightseeing, etc. And above all, she has to prove her ties to her own country compel her to return home promptly. Starting with a lucrative job which affords her the luxury of costly vacations.

An invitation means nothing, they won t even read it. She has to apply for and qualify for her own tourist visa.

If she has e-mail, send her an e-mail.

Otherwise send the letter by mail. Write: "I would love for you to visit me in the United States."

you don't ... she apples for a visit visa ..

she need to have a home, job and high income

visit visa

http://travel.state.gov/content/visas/english/visi...

Telephone? Facebook? However you normally talk to her.