Yes, it is true. You both are brother and sister but through distant relatives. So, it is like, if your family wants, you can get married but if they do not, then they can give excuses for that.
It also depends upon the way your family has maintained the relationship with that boy's family. There are still many families living together or nearby in small places. In such cases, maybe the actual could be distant but due to residing in the same house or nearby it is counted as a close relation.
Apart from the family views, you also need to consider the scientific reasons for not getting married among relatives. It is due to hereditary problems which may become dominant if make pair with the same or similar genes. For this, you should consult a geneticist who may study your family history or will conduct pedigree analysis.
Living & Learning- simultaneous processes!