I had doubts & stopped going to church a few years back. I wasn’t sure there was a God & said I was Agnostic. Things changed for me went my mum went on pilgrimage to San Giovanni Rotondo & Pietrelcina In Italy asking St Padre Pio for help. I’m not saying you need to on pilgrimage, but I do have some suggestions. My first suggestion is to buy the Green Scapula which is for conversion of sinner. Regarding this scapula our lady said. “This scapular is not like others [it is not based on a religious habit] but merely two holy images on a single piece of material. Therefore, no special formula is required to bless it or enroll someone in its use. It suffices that it be blessed by a Catholic priest and worn by the one whom we desire to benefit by Our Lady’s intercession. If, on the other hand, the person is unable, or even unwilling, to wear it or carry it, it may even be slipped, unknown to them, into their clothes, possessions, home or work environments, etc." You would say the prayer " Immaculate Heart of Mary, pray for us, now and at the hour of our death." each day for the conversion of your sister as she won’t.
Another suggestion & what helped me loads with my doubts was the book: What Happened To Me? Reflections of a journey by Randell Niles. It’s a brilliant book, is not long & I highly recommend it, for you & your mum to read & discuss before trying to come up with a plan for your sister.
I also suggest you get these trilogy of books by Taylor Marshall:
Book 1: The Crucified Rabbi: Judaism and the Origins of Catholic Christianity
Book 2: Catholic Perspective on Paul
Book 3: The Eternal City: Rome & the Origins of Catholic Christianity
The Paul Book really does show how the Catholic church is the true faith. Also please continue to pray the rosary for your sister & for her conversion. God Bless you!