Fired during pregnancy

Hi there . . . I need help! My sister is due to have her child in 18 days. She works for a large company that is based out of a different state but she works in Texasand I know state to state the laws are different. This company is a money hungry sales company who has no care in the world who they step on to make money. My sister has been performing up to the standards of the company and has been basically told if she does not reach this unatainable goal by Sunday in sales that she will be let go based on Job performance. This company I can count 5 or 6 people that I know of who have been fired during pregnangy and usually right before the person takes maternity leave. I feel confident thay are trying to fire her because they don't want to deal with the headach of FMLA and are trying to use poor job performance as the issue, but she has proof that her performance one year ago to date was exectly the same as now and what is required of her. Does she have a case, could she sue if she can back up that she indeed performing that same as before she was pregnant? Please help!