It’s not unusual in the slightest for people living together to have very similar microbiomes even in the absence of direct contact like that.

I always pose the question - your gut was sterile when you were born, so how do you think the bacteria got their in the first place? The answer is oral-fecal transmission.

You get fecal bacteria on your hands, you touch an object, someone else touches it and they put their hands in their mouth.