One of the most difficult things to deal with is family relationships.

What your parents did was morally correct—they never made your extended family feel unwelcome. On the other hand, what your extended family did was morally wrong. You should not blame your parents for upholding what is right, because they wanted you, the younger generation, to understand the meaning of proper manners.

The way your extended family behaves will have consequences for their next generation. No decent family would want to become in-laws with a family that shows such poor upbringing. If I’m not mistaken, one of the most common causes of unhappy marriages or marital failure is an unpleasant relationship with the in-laws.

~

Apart from that, there could be another unpleasant truth based on how you described your extended family's behavior. You mentioned that when they came to eat, your parents charged them 50% off and provided free drinks. This kindness could be misunderstood as a duty—something your family is expected to provide. Your extended family might have thought your parents didn’t truly host them, but merely offered them a discounted rate. Meanwhile, your parents might have believed that offering a 50% discount was already a generous way of hosting. On the other hand, your extended family may have felt that they were not being hosted at all, but were paying for the food themselves.

I see a possible mismatch in expectations between the two families in terms of Li (礼, proper conduct or ritual propriety). If the relationship between the families has drifted apart due to such misunderstandings, it’s truly a shame. Perhaps in the future, do it this way: when they come to eat, charge them the full price just like any other customer. If you truly want to host them, do it elsewhere, not at the family BKT shop, to draw a clear boundary. If, after setting this boundary, they still behave the same way, then there's no need to continue showing kindness—because it has already been taken for granted.