Parental Favouritism - long term affects?
It can be assumed that many new parents start out wishing to love and support their offspring with all they have, and the famous "We all do the best with what we know..." is sometimes very applicable here. As parents if we are aware that one of our children does perceive a patterns of possible favouritism toward a sibling, what is the best thing to do?
Firstly, many parents may hear this from their child and decide that they are not favouring any of their children - however brushing off, disregarding or ignoring this perception is not likely to put your child at ease, especially if there are behavioural patterns that may - from the outside look like favouritism. The child is likely to become insecure, disregarded and possibly feel unloved. If the situation continues to go unresolved highly negative sibling rivalry, jealousy and general dislike among family members is also a possible avenue, eventually affecting the favoured and non-favoured child alike.
Therefore it is best to acknowledge these feelings and explain why either sometimes one child needs more support than another - as there may be a very good reason why, or demonstrate how you are giving both children your time, effort, love and trust.

Unfortunately not all parents will "do better, when they know better". I know this because I have lived with this for most of my life. I have witnessed very obvious patterns of favouritism from my parents toward my sibling, and I have mentioned it multiple times in my life. My parents may have had very good reasons for providing my sibling with more support, more love and more trust, however I was kept in the dark and told that I was "imagining things" and to "be thankful for what I have" whenever I mentioned this favouritism. I tried to ignore the favouritism, I stopped mentioning it, but I couldn't completely ignore it and therefore would chat back when I was told to "be thankful". This was because although I was thankful for a roof, food and an education - it was hard to be thankful for always coming in 2nd best. This behaviour resulted in being labelled selfish, crazy and called a "drama queen" and a "bitch" by my father.
This name calling and denial of my reality began to result in a significant sibling rivalry, a teenage hatred of my father and a resentment of my mother for telling me I was "imagining things". Although I have outwardly forgiven them for these choices, it has affected my self esteem for many years and caused me to compare myself very negatively to others around me - always hoping that I would be the favourite in any new arena, (unlike in my home-life) and being upset if ever I found I wasn't the favourite.
Even to this day I am brushed off when I mention my sibling's girlfriend of approx. 2 years being offered a key and code to my parents house. Where as I am still required to keep my husband of 8.5 years in the dark. I think to myself; "Maybe there is a really good reason for this" - but when I enquire I am brushed off and told "She doesn't use it much, just look forward and be thankful" - whatever that means. This is the problem. I continue to ask myself - why was it/is it different for me?
Educator Linda J. Zervas and Psychologist Professor Martin F Sherman's 1993 study regarding perceived parental favouritism notes that parents are likely to show favouritism to one or more siblings in subtle ways and not only does the non-favoured subject enter adulthood with the lowest self esteem statistic, but that the favoured subject still has a lower self esteem statistic that subjects that indicate no favouritism from parents whatsoever (p. 25). They also remind that "Cognitive theory suggests that it does not matter if the parents really do favour one child; rather, the child's perception of favouritism that may have a greater effect on his or her development" (p. 32).
Subsequently this is my plea to parents out there:
Even if it is necessary to favour one child over another, or if you believe you are not favouring one child over another, still don't dismiss the non-favoured child's feelings - the feelings are valid. Explain your situation. Do not lie or deny their reality: " you are imagining things".
My Saving Grace:
I have spent my 20s and early 30s attempting to repair my ability to connect with others and trying to always look past the favouritism toward my sibling. But when I was young, lonely and considered problematic by my family, music was my refuge. I turned to music to fill the parts of my heart that were not filled by family ties.

I listened to pop, rock, gospel, blues, jazz, musical theatre and those artists stories and melodies are how I filled the holes in my heart. I felt a kinship with their pain, their happiness because they were consistent and non-judgemental. This is why although I have a wonderful husband and I still see and spend time with my friends and family - music will always be one of the biggest loves of my life - because it was there when no one else was.
REFERENCE:
Zervas, L, J. & Sherman, M, F. (1993). The Relationship Between Perceived Parental Favouritism and Self-Esteem. The Journal of Genetic Psychology, 155(1), 25-33. https://doi.org/10.1080/00221325.1994.9914755