Probability of carrier for recessive trait given parents are carriers and sister affected
Assuming the trait is caused by a single gene, she would indeed have a roughly 50% chance of being a carrier. If both parents are carriers but unaffected they will both be Aa (with a being the recessive mutation). For your girlfriend to be a carrier she will have to be Aa, of the 4 possible combinations her parents could have passed on (AA,Aa,aA,aa) two lead to this outcome, thus 1/2. If you are sure that she is unaffected the aa option is eliminated and she thus has a 2/3 chance of being a carrier.

If you are not a carrier but your girlfriend is it will be impossible for your child to express the trait (but would still have a 50% chance to be a carrier!). If you are both carriers you loop back to the original situation of her parents, your child would have a 25% chance of being affected and have a 50% chance of becoming a carrier. I must emphasize that real life genetic inheritance is more complex than these textbook calculations, chromosomal crossing over and whatnot can seriously alter these percentages.

I am not a health professional, consult your physician and/or get your girlfriend tested (if possible).