Thanks for asking:
Because there is but One true God; that OTG can and DOES have only one set of faith beliefs and founded, following OT tradition of One Chosen people; only One Church [Eph 4:4-8]
The problem common to all Protestant denominations is they usurp God’s exclusive Authority by demanding God effect salvation the way THEY want it or determine it to be.
God does not work like that.
It was Divine perfection that choose to found only One New Faith in One New Church. When there is JUST ONE option; it has to be the correct one. So Christ wisely gave ALL of the Keys [things needed] to ONLY and directly too St Peter and the Catholic Church. so everyone COULD know the truth and where to find it.
Salvation outside the church is conditionally possible to those “who NO fault of there own do not know of christ and His One true church.” CCC 780, 845, 847, & 1260 fully express this truth.
Not practicing ones faith is the same as denying its truth and need.
Your Aunt is not living in this example true Christian Charity. BUT going to Mass is one of the Commandments. Serious stuff to disobey.
Pray much my friend!
Patrick