Two sisters were born on the same day in the same year, yet they aren't twins. None of their other sisters share the birthday. What's going on? Explain.
When you say "two sisters", one thinks it means "they are each other's sister". While it actually only means "each one has a sister/brother". So the easiest answer is "they are not in the same family"
The two sisters aren't twins, they are either triplets (or greater) with the other triplet being a brother (or bothers if greater than triplets).
The sisters are nuns. Their shared birthday is a coincidence.
Same father, different mother.
they are step sisters. One is adopted into the family.
Two identical twin brothers marry two identical twin sisters. They each have baby girls born on the dame year-month-date. While we'd consider the girls cousins, they are genetically sisters.
Same day, same year but what about a different month? e.g. 1st January 2000 and 1st December 2000 (more than 9 months apart)
They have the same genetic parents but different surrogate mothers.
One or more has died, so you could say they WERE twins but not ARE twins.
Having sister companies founded in the same day at the same is not unusual.
It's very similar to Colonel Panic's answer:
The two ladies are nursing sisters; their shared birthday is a coincidence.
In the same vein as Taemyr's answer, but reversed:
TL;DR: One Mother, possibly Two Different Fathers
Superfecundation: Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse. The term superfecundation is derived from fecund, meaning the ability to produce offspring.
Heteropaternal superfecundation occurs when two different males father fraternal twins.