Wa `alaykum as-salam wa rahamatullah wa barakatuh
You have the responsibility of advising your sister of the gravity of neglecting prayers. If, however, a Muslim neglects the prayer out of laziness or carelessness, while nevertheless believing that the prayer is obligatory, then he/she is still a Muslim yet is religiously corrupt.
In general, Muslims today who neglect the prayer fall into this category, and so by Allah’s grace they are still Muslim. She needs to know that it is one of the gravest sins. Buraidah (may Allah be pleased with him) reported that the Messenger of Allah (peace be upon him) said, “That which differentiates us from the disbelievers and hypocrites is our performance of Salah. He who abandons it, becomes a disbeliever.” (At-Tirmidhi)
You must also explain to those who propose for her marriage that she does not pray, because it is a defect in her character. It must be frankly shown and explained to those who are interested in getting married to her.
Views expressed by hosts/guests on this program (live dialogue, Facebook sessions, etc.) are their own and their appearance on the program does not imply an endorsement of them or any entity they represent.