Islam teaches that sons and daughters should respect their parents, honor them, love them, and serve them. It is the duty of the children, whether male or female, to show their utmost love and respect to their parents under all circumstances. The parents also should be very caring and kind towards their children and treat them fairly.

Focusing on your question, we would like, first of all, to address the issue from the point of showing dutifulness to one’s parents. This duty is highly stressed in Islam. That is why we see that both worship and dutifulness to parents are mentioned in several places in the Qur’an as an inseparable duty. Allah says, “And serve Allah and do not associate any thing with Him and be good to the parents.” (An-Nisa’: 36) Allah also says: “And We have enjoined man in respect of his parents—his mother bears him with faintings upon faintings and his weaning takes two years—saying: Be grateful to Me and to both your parents; to Me is the eventual coming.” (Luqman: 14) He further says: “Thy Lord hath decreed, that ye worship none save Him, and (that ye show) kindness to parents.” (Al-Isra’: 23)
So in the light of the above, one owes one’s parents dutifulness and due respect. Even if it happens that the parents are polytheists, they still deserve due respect from their children; i.e., despite that there is no greater sin than shirk (polytheism), and despite that a child must not obey his parents if they command him to commit this heinous sin, however, he should keep good relations with them. Allah says on this: “But if they strive with thee to make thee ascribe unto Me as partner that of which thou hast no knowledge, then obey them not. Consort with them in the world kindly.” (Luqman: 15)
Thereby, according to your question, noticing some sign of favoritism or partiality in the way parents treat their kids does not justify depriving the parents of their right to dutifulness. On the contrary, full respect and honor should be shown to both of them.

On the other hand, parents should treat their children fairly without any partiality or favoritism. Delving into the issue of showing favoritism in treating children, we would like to cite for you the following fatwa:

“The Prophet (peace and blessings be upon him) gives a general order to all Muslims: “Fear Allah and treat your children fairly.” In other words, he makes fair treatment of children part of being conscious of one’s duty toward Allah and fearing Him. In another hadith, it is related that the father of An-Nu`man ibn Bashir, a young Companion of the Prophet (peace and blessings be upon him) came to the Prophet (peace and blessings be upon him) and said: “I have
given a slave as a gift to this son of mine and I would like you to bear witness to that.” The Prophet (peace and blessings be upon him) asked: “Have you given all your children similar gifts?” When the father answered in the negative, the Prophet (peace and blessings be upon him) said: “Seek some other witness for your deed, because I do not witness injustice.”
Based on this hadith, many scholars have ruled that it is forbidden for a father to give a gift to some of his children in a way that makes him prejudicial against others.
Thus, in the light of the above-mentioned facts, parents are urged not to deviate from the course of justice in dealing with their children; they should treat them fairly. This applies to every aspects of life, moral or material. It is not permissible for a father or a mother to give a child a preferential treatment to the detriment of others.
In some communities, sons are given preference over daughters. In Islam, this is not permissible. If parents provide their sons with a good standard of education they are supposed to provide their daughters with a similar standard of education. Similarly, if parents give a gift to one of the children, they should give similar gifts to the rest of the children, sons and daughters alike. Therefore, complete fairness and justice should be maintained among children.”