In England the husband of my aunt is my uncle, but in A L'Ombre des Jeunes Filles en Fleurs (Proust, vol 2 of the recherche), St Loup refers to "le mari de ma tante". Is that normal? Also he refers to "une tante de ma tante". I haven't put pen to paper to work that one out, but how can she be the aunt of his aunt without being either the aunt of his mother or the aunt of his father?
Le mari de ma tante is mon oncle, it's exactly the same thing. Saying le mari de ma tante might imply (but not necessarily) that my uncle is not a relative by blood but by marriage, in that case I could also say that he is mon oncle par alliance. In the same way I can have une tante par alliance.
The rest of your question is not really about French, it's about family ties in general, just think that your aunt-in-law (so not a sister of either your mother or your father) has an aunt-in-law, etc.