They can. They aren’t required. They just wouldn’t sit 7 days straight like Orthodox Jews would. But of course they can attend. Especially the day of the funeral. If it’s in the house they shouldn’t bring some pork chops with them lol but they could absolutely come and pay respect. Most certainly they are to attend funeral. Anyone can.

In my 54 years of life and living in two continents and being immersed in different cultures including dealing with interfaith marriages and different denominations of Judaism I’ve never heard of a family excluding someone from a funeral and shiva of their own mother in law regardless of faith. Never. Never happens.

These are some hateful folks. Well they don’t want to be at your wedding. So that’s something. I hate them because they lie about my people’s tradition to arbitrarily exclude someone. Makes us look bad.