Are my sisters really twins?

I have twin sisters who have two different dates on their birth certificate. One says March 8th 1994 and the other says June 16th 1994. The twin with the March 8th birth certificate also have HBV which my parents say was passed from a nanny. This nanny took care of all of us though and no one else was infected. They were also born in Saudi Arabia which supposedly leads the way in accurate birth certificates. Is it possible my father had an affair with the nanny without getting HBV? Is there any way to know that they're twins without a DNA test?

well, they could have been conceived together but maybe one wasn't fully developed so had to wait, if this is not true than I don't know, it takes 9 months for a baby to be fully developed and they were not born 9 months apart, have you asked your family?

edit: maybe one (or both) was adopted?

If they are twins, their birth date would be the exact same. Even the time of their birth would differ by very few minutes.