Why Is My Fully House Trained Dog Going Toilet Inside?

I had a 1year old Yorkie and Westie. both fully house trained. the westie still is the yorkie (*****) is not peeing insde… she was very good untill recently.. im not sure why… her brother goes as soon as you open the door. i take them out regular intervals and priase her whe she does but lst few weeks she has run back insde peed and coem back out! im really confused… I am currently going back to basic with puppy training and taking her out every hour and praising and ignoring when she does it inside but im a little confused why shehas reverted??
Any ideas…..
there has been no changes in the house only change is winter months e.g. weather and dark but she doesnt seem to mind is as still goes out side despite bad weather and dark but coems in to pee!
please help conused dog owner

We Currently Have A West Highland White Terrier, Aged 5 Years And Fully Grown..?

and we’re thinking of getting another dog, which ones would you suggest? we wouldn’t like one bigger than him as he would feel inferior, and we wouldn’t want that.
we realise that it is a struggle to care for one dog, let alone another, but we are fully prepared, all i’d like is some suggestions.
thanks!