I understand your concern about this relationship that your brother in involved in. The best thing will be to talk to your brother in a non-judgmental manner. Do not be critical about this lady as he is likely to become defensive. Simply ask him to describe his feelings and thoughts regarding this relationship. If you say that he does not appear to be too happy himself let him share these feelings so that he has an outlet. Then suggest ways of dealing with the situation that may include him consulting a mental health professional along with this lady so that some objective feedback can be sought with regards to their relationship.