In Reba, the star stands in for the audience and express our appalled dismay over this sordid sitcom’s premise. The Oklahoman Reba McEntire plays the Texan Reba Hart, whose shameless dentist husband (”Murphy Brown”’s Christopher Rich) has left her for his pregnant dental hygienist girlfriend (Melissa Peterman). And Reba’s unmarried teen daughter (JoAnna Garcia) is pregnant by her moronic boyfriend (Steve Howey). Reba wants us to believe that all this vulgar, selfish behavior can result in one big happy family situation. Only McEntire, still glowingly feisty from the rave reviews she got in Broadway’s ”Annie Get Your Gun,” avoids looking furtively desperate in this seamy enterprise.