Sexing bunnies

Question
I recently purchased two 9wk old bunnies from my local pet shop who assured me that they were both female.  However the smaller of the two is consistently trying to mount the other.  Having researched this a little I believe this may be a dominance issue but it is always the less apparently dominant of the two that does this.  Does this mean that this rabbit may actually be male?  I am very conconcerned as I was led to believe they were sisters and they are not yet old enough i believe to be neutered.  Is it possible for me to sex them myself?
Many Thanks

Answer
Dear Sam,

Sexing baby bunnies is a challenge, and there are no guarantees you'll get it right.  But I hope this will help:

www.bio.miami.edu/hare/sexing.html

In any case, both bunnies should be spayed/neutere, even if they are the same sex, to prevent fighting and ease the bonding process.  Females, in particular, should be spayed, as they run a very high risk of uterine cancer otherwise.  Please see:

www.rabbit.org/health/spay.html

and find a good rabbit vet here:

www.rabbit.org/vets

Hope this helps.

Dana