When discussing marriage, my mother-in-law brought my sister-in-law to meet me, and one form of address made me call off the engagement immediately

(From a reader submission, images in this article are from the internet)
When discussing marriage, my mother-in-law brought my sister-in-law to meet me, and when I heard how she addressed me, I immediately called off the engagement
Recently, my boyfriend and I had been preparing to discuss marriage, and were starting to negotiate and prepare various aspects. The two families had met not long ago, and the parts about betrothal gifts and dowry had been mostly settled. But after my mother-in-law brought my sister-in-law to meet me, because of one form of address from her, I immediately called off the engagement right then.
I dated my boyfriend for 6 years, and we met in university. His family circumstances were ordinary, while my family was comfortably off. Having grown up carefree since childhood, I was very happy with his straightforwardness and honesty, and did not mind it at all. Because he treated me very well, and we were also young, with limitless possibilities in the future, a temporary financial difficulty was nothing. I firmly believed that as long as we worked hard together in the future, everything could be obtained with our own hands.
Later, after we had been together for more than four years and had also graduated from university, he told me something else, which surprised me a little. He said he was not the only child in the family, and had a younger sister below him, with a very large age gap. At the time, he and I were both 24 years old, but his sister was only a little over 6. After hearing it at the time, honestly, I was also a bit surprised. There was nothing else to it, I just felt that his parents wanting to have another daughter after their son was already so old was a bit unwise, especially since the family circumstances were not very good either, so it seemed unnecessary. But he repeatedly told me that after he started high school, his parents felt that once a son grew up he would leave home, and they were bored at home, and it just happened that she became pregnant, and did not want to abort, so she gave birth.
In the end, this was also another family's private matter, and I had no right to interfere. I thought that in the future I would only be living with my own husband, and at worst my natal family could pay the down payment on a new home for us so we could live separately from my parents-in-law and sister-in-law, and everything would be solved, so at the time I did not take it to heart. Unexpectedly, when it came to the stage of discussing marriage, a few days ago my mother-in-law brought my future sister-in-law to meet me, and this little girl opened her mouth and called me mom. At the time I was extremely astonished, and even misunderstood, thinking that my boyfriend had gotten into some trouble in high school and was using his parents' name to deceive me. Seeing my misunderstanding, I relentlessly questioned my boyfriend. My prospective mother-in-law hurriedly explained that she felt she and my father-in-law were getting old and did not have enough energy, and wanted me to help take care of the child after we married, which was why she had the sister-in-law change how she addressed me.
I think my mother-in-law was a bit too fanciful. As a girl who had never been a mother and had not even married yet, I did not want to suddenly have a "daughter" right after getting married, and this was not within my obligations either. My mother-in-law was very angry, saying that if I wanted to marry into her family, I had to help raise my sister-in-law, otherwise there was no discussion, and my answer was to immediately call off the engagement. Everyone, tell me, was this not an overly unreasonable demand on her part?