Maybe due to these marriage rituals that include Genesis 2:24, people in that era were more concerned with the religious union than the civil one. As Lady Cat became one flesh with her husband, his grandfather is her grandfather etc.

I truly have no idea how people spoke back then and what customs they used to define their relationship. Does Georgiana say she always wished for a sister or for a sister-in-law? When you got married, you called your MIL "Mother" or "Mother-in-Law"? Did you always clarify and quantify things, e.g. "my husband's mother" or "my mother in law".