How can one find information about in laws from a horoscope?
Astrologically one can find the status of in-laws and relation with them. In astrology, 7th house rules marriage and spouse. So counting those many houses from the 7th house would result in those relationship.
Mother-in-law: The 10th house which is 4th house from 7th house represents mother-in-law. More the benefics or benefic aspection to 10th house indicates a cordial relationship with mother-in-law
Father-in-law: The 3rd house which is 9th house from 7th house represents father-in-law. More the benefics or benefic aspection to 3rd house indicates a cordial relationship with father-in-law
Brother or sister-in-law: The 9th house which is the 3rd house from 7th house rules younger brother or sister-in-law. The 5th house which 11th house from 7th house rules elder brother or sister-in-law.
Applying the above rules one can find any relationship with spouse family members.