Bear in mind that when people tell you such things as "it was God's plan" it is not because they know you are an atheist and they are taking the opportunity to deliberately annoy you. Many of them would be considerate to refrain from making that kind of statement if they knew that you don't believe it.

Beyond that, your mother-in-law had other people who loved her, friends and relatives who have also experienced a painful loss, and they probably do not want to get into a theological dispute at this time. I am always willing to discuss religious issues with people who want to discuss those issues. If they don't want to, I am not going to insist on it.

To possibly give you a different perspective on this, do you get angry with people who wish you a merry Christmas? You could always reply, I am not a Christian and I do not have a merry Christmas! But then, people who wish you a merry Christmas have no harmful intention, it is an expression of friendliness, even if it carries a religious tinge. I certainly have no need to get into an argument with everyone who wishes me a merry Christmas. I am already misanthropic enough.

How do you feel about someone who asks you what your astrological sign is? That is equally stupid. Nonetheless you would not necessarily want to get into an argument every time you are asked that question. It is simple enough to answer the question. I can admit to being an Ares without admitting to a belief in astrology. Of course if someone matters to me, I might want to talk about astrology and about pseudo-science in general at greater length. I do tell people what I believe, but I don't tell it to everyone I meet at every occasion.

The world is full of deluded people, which is unfortunate. Almost everyone in the world has at least some mistaken beliefs. If they do not believe in God, in the afterlife, in astrology, or that the number 13 is unlucky (etc.) they may still believe that human beings only use 10% of their brains, another absurd misconception that is widely circulated. Or a thousand other falsehoods. So, we can struggle to educate people, but we do so knowing that there is only so much we can accomplish in a world that is drowning in ignorance and misinformation. I would just urge you to remain calm about it.