In Polish scientific literature the question of terminology of kinship, consanguinity, affinity and other family relationships in medieval sources was raised twice. The in-depth analysis of the issue was presented in the monograph of a linguist Mieczyslaw Szymczak (1966) and the article of a medievalist Maria Koczerska (1982). Both authors emphasized the temporal variability of terminology and its ambiguity. Thus one could say that the issue of genealogical terminology in a medieval Poland is insightful and well researched, and the newer studies may only add illustrations and examples. J. Bieniak - a genealogist, states that the structure of Szymczak's study is inconsistent and erroneous in many respects, especially in case of full and partial family relation. The partial family relation includes stepsiblings, as well as stepparents and their stepchildren. It may be called 'foster relation'. Bieniak states, that the differentiating full family relation from partial family relation in the fourth degree of kinship is pointless, as in the fourth degree of kinship there is no common parent. Additionally, the relation between the stepmother or stepfather and the stepchildren is no 'foster kinship', but one of the classical forms of family connections (as a consequence of a marriage of ancestors in older generation). Thus, the future genealogical monographs of the family relationships terminology should not reproduce that artificial question of 'incomplete family relation'. The author draws his attention on the double meaning of family relations terminology that is present in historical sources, as it designates not only real family relations but also is used in conventional way. Bieniak ponders the problem, when such a terminology was used to express a real family relation, and when to express a conventional one (i.e. the usage of such terms as 'father', 'brother', 'son', 'sister' etc. in convents, monasteries, but also at the court). The author also considers the problem of evident errors in the historical sources, which - afterwards - became a part of historical knowledge as recognized, true and indisputable facts. He uses two examples to present his method. He states, that historian should: 1) find inconsistency of sources, 2) compare the sources and explain which sources seem to be true and show the real genealogical family connection, 3) explain the cause of the error.