Granted, Yusuf is a somewhat common name, and I guess it's always possible to find someone from the same race who looks very close in looks to someone else. Still...HRM. LE GASP.
Also, Poland? For some reason, that makes me laugh-but it may be because I've watched too much Hetalia.
First off, I'd like to stay I love this comic. Second off, after seeing the DNA test, Mary is assuming that they aren't siblings. Most likely one or both could be adopted, but it is likely Yusra, since nobody in her family from her homeland would take her and her brother because she is not blood. But it is a theory and it may or may not be true.