Re: Is it possible for my younger sister and I to be twins?

Dear Elizabeth,
No it is not possible for you and your sister to be fraternal twins.  
While some techniques have been recently developed to deliver one twin and 
have the uterus retain the other twin for a few extra weeks for further 
development, this requires carefully designed intervention.  An interval 
of two years would be impossible even with strong intervention.  The 
explanation is that siblings can, because of the specific sperm and egg 
that actually get together at fertilization, be very different or very 
similar, almost like twins.  Of the millions of sperm that your dad 
produces every day there are many sperm that would be almost identical to 
each other in DNA content and many that would be extremely different from 
each other.  In the same way, the several million eggs that your mother 
had in her ovaries as a fetus and neonate would have contained similar and 
dissimilar DNA content as well.  Even though girls don’t make any new eggs 
after about the 5th month in utero, there is still the possibility that 
two very similar eggs were ovulated two years apart and just happened to 
be fertilized by two very similar sperm on those two cycles.  This occurs 
every now and then and you and your sister are just one of many examples.  
This would explain your similar likes, developed habits and probably 
dislikes as well although you don’t mention dislikes.