I think you have to look at the reasons for different treatment rather than just going 'they're being treated differently, that's unfair'.
Take giving birth for example, I want my mum there because she's there for me, she's loved me and raised me and although I wouldn't just strip off in front of her I don't feel uncomfortable with her seeing me in a state of undress, in pain, legs in the air and possibly shitting myself. My mums there for me whether MIL is there primarily for The Dad and the baby and as the dad isn't the one giving birth it's not about him and until the baby's out it's about the mum, not the baby.
Same for coming to the house. When my mum comes over she helps me with things, yes she's a guest but I don't treat her like one, I don't need to clean for her or worry about being presentable and tidy or not walking round in my dressing gown whether when MIL and FIL come I do. MIL is also much more critical (only of me, sexism much
) if the house isn't clean and tidy whether my mum doesn't care.
Similarly with being 'allowed' to do task. When my mum, for example, helps with the washing she just gets on with it, sits with me whilst we fold clothes and chat about other stuff, I have no problem with her seeing my underwear because she's my mum. Whether with MIL when she has helped with washing in the past she comments on my clothes, my underwear, how I wash and fold things ect. Also as my MIL I feel uncomfortable with her handling my underwear, whether DP has never expressed a problem with my mum handling his and if he did I'd respect that, still might get her to help me with washing but only my stuff. If only DH is in and MIL comes round I have no problem with her helping with washing as long as it's not my intimate clothes, and DH knows this, because I don't have to listen to her criticism, but he knows and respects my boundaries.
So yes it is one rule for mym one rule for MIL but it's for good reason, because they have entirely different relationships with me.