Faithfully, I feel for your FIL!
I mean, just put yourself in his shoes for a minute! He, for whatever reason, really dislikes the Catholic Church, but his son finds a …a Catholic wife.
Then, to make matters worse, his daughter also marries a Catholic man (even if not practicing). Poor man, he constantly needs to explain himself, “No, my son is not Catholic, he is only getting married in a Catholic church because of his wife”, “No, my son-in-law is not Catholic either, despite his fondness for rosaries”, LOL! Poor man, what a hard life!
Well, if only he would face his bigotry once, but let’s not expect miracles. Anyway, it could be that he is not against God, he is merely against organized religion, chiefly against the most visible organization which is the Catholic Church, but he might be against other organized religions (Baptist, Methodist, whatever) as well. His wife might have had an influence on him? I’m guessing. Native Americans were apparently dealt many unjustices here in the USA, and I heard a Christian missionary who is Native American, say, that 99% of his people here in the USA reject Christianity because they have been mistreated by Christians. I remember when I traveled to the Smoky Mountains, the story of the Cherokee Nation being uprooted and forced to move to Oklahoma, a move they remember as the “Path of Tears”. Then, in the area of Monument Valley (Arizona), and Canyon de Chelly, I’ve seen documentaries remembering the genocides committed against Native Americans (Navajos) there. It almost seems like, wherever I travel, I hear more disturbing stories. On a certain level, I’m not surprised that Native Americans reject Christianity. Even down in Mexico, in 1531 when Our Lady of Guadalupe appeared to the Aztec Indian Juan Diego, they were on the verge of “civil war”. The Spanish conquerors treated the Aztecs very badly, and very few Aztecs embraced Christianity. In fact, they planned to raise up and wage all-out war against the Spaniards. But the prayers of the local Catholic Bishop Zumarraga, who has been praying for Our Lady’s intercession, for peace and the conversion of Aztec Indians, have been heard. In essence, it took a miracle, the miraculous apparition of Our Lady, dressed as their own, appearing to one of their own people, and speaking their Nahuatl language, to convert the Aztec Indians! But I digress. There may be historic wounds, historic sensibilities, which are influencing the attitudes of both your MIL and FIL.
Probably best to do what EasterJoy says, preach the Gospel without words, through actions.
I wonder, though, has your MIL (or your sister-in-law, for that matter) ever heard of the story of Our Lady of Guadalupe? She appeared in 1531 to a poor Aztec Indian, and sent him to Bishop Zumarraga, with the request to build a chapel in her honor, at the site of her apparition (which was Tepeyac Hill, where previously the Aztecs worshipped their pagan gods and practiced gruesome human sacrifice to the idols). When the Bishop asked for a sign, Our Lady told Juan Diego to gather the Castilian roses miraculously blooming on the hill (in the middle of winter, in an arid region that never had such roses whatsoever) and bring them to the Bishop. He brought the roses to the Bishop, and as he unfolded his tilma (cloak) in which he gathered the roses, there was the famous and beautiful picture of Our Lady, impressed on that tilma (cloak) woven of rough cactus fibers. Sorry if you already knew the story, maybe there’s someone else who never heard it! It was after this miracle, that some 9 million Aztecs converted to the Catholic Church, within less than 10 years.
Maybe you could share this story with your SIL?
It had a major influence on me, when I first learned of this apparition. I was searching for some supernatural meaning of the life, something that’s not destroyed by death, and I was dabbling in yoga, Buddhism, a bit of the occult. When I learned about the story of Our Lady of Guadalupe, I understood that God exists, the supernatural exists, and God didn’t abandon us here to our own devices. He sent Our Lady, to help the Aztecs find him, and to help them find the visible organization God instituted for our salvation, which organization is of course the Catholic Church.
With Protestants who are anti-Catholic, sometimes I think they frenzied themselves into animosity, without any real reason. I tend to think this might be the case with your FIL? With your MIL, the special circumstance of her Native American heritage could explain why she believes in God, but rejects Christianity.
Honestly, my first reaction was puzzlement. I don’t think religion is really part of their lives at all, so why would they bother having the baby baptized? I’m guessing it’s a combination of force of habit/family pressures. As a previous poster said, I guess that they see it as something people are “supposed to do” without really considering that there is a deeper meaning to the religious ritual. I’m guessing there are similar factors at play in the wedding the OP described.
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they are very nice people, but you wouldn’t want to see their reaction to a question like that! 
). Sometimes laughter. It probably makes them mad, but sometimes they say things so far fetched that all I can do is laugh… surely they aren’t SERIOUS!!! That was a joke, right???
Well, in some ways… This girl has lots of learnin’ to do! Biting my tongue among other things!