I have got some words in my lessons, which cannot be found in my vocabulary such as hijo/hija, alberca, madre etc.
Right, now I understand. I had a look and I noticed that "alberca" is part of the Household category and that "hijo", "hija and "madre" are part of the Family category. These are categories that you haven't unlocked yet. It is possible that the program shows you new words before you have unlocked the categories that they belong to. These words will likely be added to your dictionary once you have learned them as part of their category.