Something that I never resolved and we've been married over 25 years. They were adamant they were Mum and Dad, I was adamant they weren't.
I got around it by not addressing them by name at all which could be awkward if FIL answered and I wanted to speak to MIL. Phrases such as, "is the boss there" got trotted out. When talking to other people about them, I say DH's Mum... Which can get really confusing when I'm out with my SIL's DH or talking to friends of SIL
When DD1 was born, I just started using Gran/Grandad - I guess there is now an uneasy truce but it still niggles (and has become a bit of a joke amongst DH's siblings partners.)
My parents were brilliant, they asked DH if he'd rather call them Mum/Dad or by their names. He opted for the later but still won't fight my corner with his parents.
I think that, with hindsight, I should have just referred to them to their face as Mother-in-Law/Father-in-Law. "Would you like a coffee, MIL?", "FIL could I speak to MIL".
(Apart from this one huge (for me anyway) issue, they are absolutely wonderful and we get on really well)