1, your family and your sister's classmates and friends should unite and persist in opposing it. If you persuade someone to make peace, your sister will certainly listen to the peacemaker. This is a woman's normal psychology. You can even put some pressure on your sister to let her know that if she marries Xiao Li, she will end up alone and no one will help her in the future.
Put pressure on Xiao Li to stop pestering your sister! For example, return all the fruits they sent and tell him not to send these rubbish. Anyway, they all insult you like this. Why should you be polite to them?
Let your sister read more emotional books to help her get out of the strange circle.
Of course, this is not easy. I also know that I have experienced this cycle before. My ex-boyfriend's family opposed our marriage. They said too much to me. They told me to give up my stable job, and I followed their son to work hard in other places. I didn't buy a house or give anything, otherwise I would have forgotten. I regressed again and again, put down my dignity and persuaded my parents to accept the conditions. It turned out to be useless, and they would only push their luck. Later, we quarreled for two months and got hurt deeply, but we broke up. This can only be done slowly, and you have to be completely injured to wake up.