Non-Muslim In-Laws Trying To Take My Children Away From Islam

Assalamu Alaikum,

Please bear with me as I have never created a Reddit post before, but have been struggling so much lately with the weight of what is happening that I felt a desperate need to seek support and advice.

My husband of thirty years is a white American Muslim convert, who converted before I met him with the help of a friend. I am a born Indian Muslim and very devout. My husband has always kept his conversion hidden from his mother and father (this is the only family he has - no other relations, siblings, cousins, etc.) because when he first told them of his plans to convert all those years ago, his mother went into extreme shock, could not process it, and had two successive heart attacks within a fortnight and needed surgery. They are devout Christians. He had no option but to hide it, for the sake of his mother's health, especially as his mother's doctor said she must be kept away from shocking news to avoid any further risks to her health. His mother's health has been very weak ever since. His parents live in America and we live in Canada, so hiding his faith has been easy since they aren't there to see him pray, fast, etc. They did not like it when he married me, a Muslim woman, but since they did not know their own son was also a Muslim, they didn't push back.

Fast-forward thirty years - we have two children in their 20s (one son, one daughter) and their paternal grandparents are trying to lead them away from Islam. We raised them as Muslims, which was not easy given my parents in-law, but after many fights with me over the years (wanting to take them to Church service and me refusing, etc.) they accepted that they could not fight me and that I was raising them Muslim (obviously they did not know of their son's involvement - they just thought that as the mother I naturally had more faith influence on them over time than their son, and that that is why they turned out Muslim).

The issue is that they have put the thought into my children's minds that they should be free to marry whomever they want, even if it is a non-Muslim - because they 'realised' that this is the way that they can 'win' in the end. They talk about this every time they see them, and it has (without me realising sooner) already turned my daughter's and son's hearts towards that idea. They were raised devout Muslims and are, but they are still young impressionable kids in a world full of attractive non-Muslim men and women, so I am struggling to emphasise the importance of marrying Muslims to them without being met with eye-rolls and "but grandma and grandpa said it's okay".

I have tried to rally my husband to try to curtail their influence, but to my shock and horror he said that he spoke to them and he now also does not see the harm in it. He has always been a practising Muslim, but after years of struggling to feel truly 'included' in the community, his heart seems to have hardened towards some parts of the community, and also his parents seem to have poisoned his mind over time.

I don't know what to do. Please help me. My marriage was smooth up until now, and our children were raised with the best Muslim education we could possibly provide. I stopped my career to work full-time on raising highly educated Muslim children, but it feels like I am about to lose everything I worked for. I am trying to pray dua and have tawakkul but I am failing.