First off, I am not aware of any source that makes a claim that she had a low UGPA or LSAT score. What I am aware of is that prior to law school, she had some serious jobs with serious credibility.
She "worked for the State Department for five years and the U.S. Agency for International Development between 1989 and 1993."
That probably helped some as well. Plus, when she went to college, she wrote a senior thesis. Did you? I sure didn't.
That said, yes - being the child of an influential person can help get a person into good schools. It makes sense that prestigious schools would want prestigious alumni.
Of course being Cheney's daughter gave her more of a boost than being a woman would. As to the speculation that she was a C student, that's likely pure exposition based on political affiliation more than proof, logic, or reason. The University of Chicago is a fine law school and I don't think she would have graduated and been able to do so much if she was marginally acceptable, despite her famous father. Try to remember that for as many people who might consider the Cheney name an asset, there are also plenty who would see it as a liability.
For the record, I'm sure Bill Gates' children will be able to get into whatever school they want as well.