Assalamualeikum. Is it haram to let my wifes family visit us with their dog(pet). (Her father is a nonmuslim) and they cant visit without The dog (because they cannot let it stay Home alone). It is my wifes dog but she left it with them when marrying me. My wife tells me that she cant refuse to let them visit even with the dog. Is it haram for me to let them visit my appartment with the dog?
Wa alaikum as salaam
It is not allowed to have a dog as a pet unless they are a working dog in which case the dog has to be kept outdoors as their saliva (when wet) is impure, the reward is diminished daily and it prevents the angels from entering the home where the dog is kept. It is also ok for you to visit one's home where they have a dog as a pet. This link should clarify that further.
Now, as to a dog frequently entering the building, rather than living there. In earlier reports (before doors were on the mosques and before the ruling of "cleaning the vessels a dog licks" people were still able to pray in that building as it wouldn't make the building impure that a dog passed through. This summarizes the following hadith:
Al-Bukhaari (168) narrated that Ibn ‘Umar (may Allah be pleased with him) said: I used to stay overnight in the mosque at the time of the Messenger of Allah (blessings and peace of Allah be upon him) when I was young and single, and dogs used to urinate and come and go in the mosque, and they did not sprinkle water over any of that.
However, in regards to the above ruling any rulings that came after that should now be followed and that ruling will be abrogated as this happened prior to the current rulings we are expected to follow
And Allah knows best