genetics - Probability of carrier for recessive trait given parents are carriers and sister affected

I'm trying to understand more about inheritance of an autosomal recessive trait that runs in my girlfriend's family. How could I start looking at this?

My girlfriend's sister, C, is affected with a recessive trait. Her parents, A and B, are confirmed unaffected carriers. Am I correct to assume my girlfriend, D, is a carrier with probability 50%? Or must I take into consideration that we know the state of C given she is affected.

Furthermore, if I (E) have a child with my girlfriend, what is the probability that our children are either carriers or affected? My carrier status is unknown.