So our 11yr old twins have been going with their nan ( husbands mum ) on a Saturday for a while. In saying this the past 6 or so times she's been bad mouthing me and calling me names th3n telling my kids to not tell mem

When I really think about it she's been doing this on and off for over 5 years !!! As far as I'm aware I've done nothing wrong, she'll be nice to my face then horrible once she's left. I've forgiven her for it multiple times and the husband has spoken to her about it but she just gets angry. I'm at a point that I've really had enough and I'm finding it quite hard to forgive her this time. What do I do she's not approachable, and always turns things around and twists words.