If a girl is certain that her father will reject a proposal from a pious and compatible boy due to non-Islamic cultural reasons, can they get married without informing him to avoid committing a sin?
Bismihi ta'ala
If she does not want to approach her father about this, she must try to find alternative ways to inform him and get his approval, like through elders, local scholars, etc.
We must remember that in marriage, there are many surrounding factors that make it successful. One of them is harmony and blessings from parents.
And Allah knows best