Why are our mensturation cycles synced in our house?

May or May not be. Menstrual synchrony (the mcclintock effect) is a phenomenon 1st reported in 1971. It's existance has not been definitively established, and studies investigating it are controversial. Those who claim it is real state it is due to pheromones, hormone signals, between women. Others claim it is just a random occurrance. Some studies disprove it, others state it is possible. It's still a "mystery".