Mother in Law advice

I moved into my fiancé’s house 4 years ago. He previously lived alone and his mother would come over during the week when he was at work, she often cleaning and did washing for him. In the last four years she and his father still come over during the week while we are both working they always call the night before. I have always hated the idea of her being in the house without us but as she is in her 80s and it is his house I let it be. Until today. After a very long week at work (60 hours plus) and as a result a very untidy house I arrived home to the smell of bleach. I was utterly mortified, our house was not just untidy but I had left a pregnancy test (negative) in the bathroom. I was seething when my partner got home. She phoned our house an hour after we arrived home to scream done the phone at us about the state of the house, my partner couldn’t say anything as she was shouting too loudly. She ended up hanging up mid conversation.

So my question is, considering her age, how do I approach this? Is it appropriate for me to find this unacceptable? It is his house but also my home.