Unvenial

"I wasn't aware that going to the park in the middle of the night was that much of an unvenial sin. When my mom found out, she..."

What do you guys think of the word 'unvenial' and its usage in this context?

A venial sin is one that is easily forgiven. So it means that going into the park is a serious sin. Nothing particularly surprising usagewise.

I have never seen "unvenial" afaik. I am not Catholic, but wouldn't a Catholic say "mortal" for "unvenial"? Or is this a subtle way of saying "serious" sin?

I think it's meant to be funny.