Twin Sisters

The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week.
In very rare cases, twins become conjoined twins. Furthermore, there can be various degrees of shared environment of twins in the womb, potentially leading to pregnancy complications.
It is a common misconception that two placentas means twins are dizygotic (non-identical). But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.