I believe the general practice in the Slutsk cemetary was to bury men in the men's section and women the women's section, wherever there was the next space. In the one case I have seen in my family where a man was buried next to his brother in law, that relationship was stated in the Chevra Kedisha record (these are the husbands of my great grandmother's two sisters):
2650. On Friday the eve of the holy day of Shabat the ninth day of Tamuz (21 June 1907) passed away the elderly mh"r r' Y' LEIB the son of M' SHMUEL LIFSHITS IVANER and he rests by his brother-in-law mh"r ELIYAHU who passed away on the seventh day of the aforementioned Adar (21 February 1907) row "six" by the "geonim" to the town side.
In cases where no relationship is stated I think the working assumption is there was no family relationship.
