All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible for a Muslim woman to allow her father in-law to visit her and stay with her even if her husband is absent; for more benefit please refer to Fatwa 84143.
The fact that this father in-law does not perform the prayer or that he believes in astrology is not a sound reason not to allow him to visit and stay with her. However, if she is harmed by his stay, she has the right to refuse his stay with her.
Finally, it should be noted that this father in-law must be advised about the danger of abandoning the prayer and believing in astrology should be clarified to him.
For more benefit, please refer to Fataawa 87319, 84390 and 82923.
Allaah Knows best.