I think that merely living together is not enough to make cycles sync. You have to have spend time together and interact. In my late teens I went to a two week camp. I had just finished my period when I left home. By the beginning of week two, most of the females at the camp were menstruating, including myself. 20 women between the ages of 12 and 25 from all over the country found a similar rhythm in one week. The males of the camp were a bit perplexed.
When I lived at home I never really synced with my female family members. My sister was always at her boyfriend’s house and my mom and I did not get along so we didn’t spend much time in the same room. My mom was also dealing with painful cysts at the time.
My theory is that it has something to do with pheromones. You need to be exposed to them long enough, or in higher concentrations for them to have an effect on your cycle. I also wonder if the age of electronic devices has an impact on the ability of women who live closely together to synch their cycles.