What is a comfortable English translation of 姐夫
The Chinese word 姐夫 is both a referent term and an address term. You can refer to someone as your 姐夫 (e.g. 我姐夫是中國人）as well as call him 姐夫 （e.g. 姐夫，你好嗎？）. 

The English equivalent "brother-in-law", however, is only a referent term. You can say "Peter is my brother-in-law" or "my brother-in-law is Chinese" etc., but I have never heard of anyone using "brother-in-law" as an address term (as in: How are you, Brother-in-law?) 

Here you are looking for an equivalent of 姐夫 as a direct address. I suppose the best way is for her to use whatever a normal English speaker would use to address his/her brother-in-law. In my understanding, this would be his given name or some kind of nick name that the brother-in-law would like to be called by. Addressing somebody as "Brother-in-law" would sound odd, IMO.