Just that really.
My Mum has been in a new relationship for 3 years. He has 3 dc. We're all adults so they aren't stepsisters/brothers.
But referring to them as my mother's partner's children seems such a mouthful.
One of my friends said I should just call them Stepbrother and sister, as that explains it, but that feels weird.
Anyone else in a similar situation and what do you call them?