How to treat the behavior of parents-in-law favoring big sister?

In-laws prefer older sisters, which is often encountered at home. After all, your sister is your own daughter, and you are an outsider in their eyes anyway. Generally, parents-in-law with high emotional intelligence will make an image of defending their daughter-in-law, while secretly favoring their daughters, which is acceptable to most daughter-in-law. If your in-laws are obviously biased towards your elder sister-in-law, it is not too much. For the sake of family harmony, I suggest not to care. But if the eccentricity is excessive and there is no bottom line, then it is still necessary to argue. The key is to combine specific things and not generalize.