My mom's side of the family is Catholic and such was how I was raised (Dad's side is Lutheran). I later came to be Agnostic, but still acknowledge and respect the different faiths of the world. Anyways, it comes from my understanding of the Catholic faith that one's goal in life, as a good and loyal follower of the religion, is to receive as many of the 7 Holy Sacraments as possible. These are Baptism, First Communion, Conformation, Penance (Confession), Holy Matrimony (Marriage), Holy Orders (Ordination or becoming a Priest) and Anointing of the Sick, more commonly known as the Last Rites.
By receiving these Sacraments, they are becoming pious and shedding their sins in order to be deemed worthy enough to receive their ultimate reward, which is to enter the gates of Heaven and be with the Almighty.
The family was only doing what they believed would aide the soul of their young son, and help him to be deemed worthy to enter into Heaven. They were only doing that which they felt needed to be done. In their eyes, it was the right thing to do, and the only course of action they saw to be justified.
I understand that you feel that they were bringing religion into the situation just to be annoying and whatever else you felt they were, but some people do believe so strongly in their faith that it just comes off as such sometimes. You might need to be less judgmental and more accepting of others. I know, I sound like I am preaching the Bible here, but IMO, if we all just stopped fighting over things we don't understand about each other, things that are seemingly fearful about them, and actually take the time to learn about why they are they way they are, then maybe, just maybe, the world would be a better, and more understanding place.
“The day the power of love overrules the love of power, the world will know peace.”
― Mahatma Gandhi