What do elements in the same family generally have similar properties?

Elements in the same family belong to the same group.

A group on the periodic table is the vertical arrangement of elements. Elements in the same group have the same amount of valence electrons in their outermost shell. We know that the valency of an element dictates its chemical properties. In group I for example, elements here have just a valence electron, this makes group I elements readily lose their electrons to achieve a noble configuration.

Physical properties to an extent also varies and progresses down the group steadily.

This is why elements in the same family are said to generally have similar properties