While looking for a match for marriage, if one person is Thalassemia(Carrier) and the other is NOT, there should be no problems. The only thing which you should make sure is that both are NOT Thal(Minor) - which might lead to a situation in future, they might end up with a kid who is a Thalassemia(Major) - Which is bad.
Remember, Thal(Minor) is NOT a disease. It's just a gene which can be passed on to the next generation. Like one gene decides what color hair the baby might have, the other decides what skin type will the baby have. Similarly, Chromosome # 11 and # 16 decides if the baby will have normal globin producing gene or a defective one(in case both parents are Thal{Minor})
If you brother is NOT a Thal(Minor) - do NOT worry about your brother's to-be-wife having Thal(Minor). The only thing that can happen{worst scenario - is that the kids might be Thal{Minor} - which you again want to make sure when their age comes and when they get married - that they do NOT marry a Thal(Minor)
I would also ask the girls side of family to be screened, if there are any more Thal(Carrier) in their family. So, that they can prevent a marriage between two Thal(Minor)'s
-nb