While one cannot help their feelings, the role of any parent is still to treat all their children (bio and step) fairly, and not exhibit blatant favouritism. The OP is imho exhibiting favouritism of putting her bio child above her step child, which one study I’ve already posted warns causes enduring relationship issues in stepfamilies. But here are a few more blurbs written by social and clinical psychologists explaining why favouritism is bad and unfair in the step/bio child context.
“A large proportion of parents consistently favor one child over another. This favoritism can manifest in different ways: more time spent with one child, more affection given, more privileges, less discipline, or less abuse. Research by sociologist Jill Suitor examines some of the causes and consequences of parental favoritism, which occurs in one- to two-thirds of American families.”
“Other reasons for parental favoritism most of us would judge as unfair, yet they don't surprise us much. Parents might spend more time with and feel closer to same-gender children than to opposite-gender children. In mixed families, parents favor their biological children over step-children. In patriarchal cultures, parents simply favor boys over girls.”
”Unfortunately, the consequences of parental favoritism are what you might expect — they're mostly bad. Disfavored children experience worse outcomes across the board: more depression, greater aggressiveness, lower self-esteem, and poorer academic performance. These repercussions are far more extreme than any benefits the favored children get out of it (negative things just have a stronger impact on people than positive things). And it's not all rosy for the favored children either — their siblings often come to resent them, poisoning those relationships.
Many of these consequences persist long after children have grown up and moved out of the house. People don't soon forget that they were disfavored by their parents, and many people report that being disfavored as a child continues to affect their self-esteem and their relationships in adulthood.
To make matters worse, parents are even more likely to play favorites once their children are grown up, sustaining the toxic family dynamics (e.g., bad feelings, sibling resentment).”
^www.psychologytoday.com/us/blog/the-narcissus-in-all-us/200901/when-parents-play-favorites^
“Children instinctively depend on their parents for love, care and support. Kids get motivated when parents encourage them and get demotivated when they fail to do so. Many a times, parents give undue attention to the younger child while not giving enough attention to the older ones. In mixed families, parents favour their biological children over the step-children while in patriarchal societies, parents display extreme affection for the male child.
Parental favouritism of one child over the other affects their emotional well-being. It is very likely that in their growing years, a neglected child could lose self-esteem, become depressed and also lose faith in themselves. This in turn affects their studies and intellectual growth.
So parents need to be extremely cautious and treat all the siblings equally. So let’s look at some ill-effects of parental favouritism.”
“Usually, children who are petted a lot, become spoilt brats. They can display unnecessary tantrums, demand a lot of gifts, and also display adamant behavior right from their childhood. Favoured children may feel a sense of superiority and tend to believe that they can break rules. This can adversely affect their ability to sustain relationships, along with their behavior at school, work life, and even friendships.”
“As a parent, you may be inadvertently fostering rivalry between your children by displaying favouritism. The child, who is shown less favouritism, is likely to instigate rivalry with his sibling. In the growing up years, a jealous child could even try to harm or cause an injury to his/her sibling. Parents need to understand that all children need equal attention and love while dealing with their children.”
“Displaying partiality towards a child can cause unrequited stress on the other child. The feeling or perception of being the less favourite can hurt the self-esteem of a child. For instance, labeling one of your children as smart or intelligent may lead to unnecessary and unhealthy competition among the kids, where one would continuously try to put the other down. In adulthood, the less favoured child may still lack self-esteem and hence may not be able to perform well in their work lives.”
“People usually don’t forget that they were not treated fairly by their parents. The neglected children may develop hatred towards the parent who displayed favouritism. Also, such children are more likely to exhibit aggression and inappropriate behavior in their schools and with siblings. The lack of parental affirmation and affection may leave a void in their lives which can never be filled. Children can also exhibit signs of depression very early in life.”
“As a parent, when you favour a child for being talented, this automatically makes the less favoured child suppress his/her talents. Due to lack of encouragement and support, they start doubting their abilities, suppress their talents and fail to achieve their true potential. According to psychologists, an emotionally healthy child will want to display his or her talents and will also learn faster. Always promote your kids to display their talents and do not praise or encourage only one child. Encourage all the kids to perform to the best of their abilities.”
“At social events and gatherings, children are expected to display a certain demeanor. They are expected to wish and greet the elders and also to respond to conversations. The unfavoured child may shy away and may not respond very well. As these kids grow up they tend to lack social skills due to lack of self-confidence. According to experts, unfavoured children may carry the belief that they are not good enough throughout their lives.”
^www.educationworld.in/parental-favouritism-harmful-effects-on-children/^
“When favoritism morphs into abuse, the health of the family and the psychological well being of all its members is jeopardized:
Favorite children grow up with distorted, inflated views of themselves. They are vulnerable to feeling entitled and believing that rules don't apply to them. They are likely to struggle with intimate relationships. Additionally, they are likely to grow up alienated from their siblings.
Unfavored children grow up with distorted, negative views of themselves. They are vulnerable to feeling defeated, believing that hard work and determination will not reap the rewards they desire. Depression often accompanies this experience.
Spouses observing their mates inappropriate attachment to one child are likely to be uneasy: either they are jealous of their child's relationship with their spouse or they are relieved that the favorite child is filling a void in their spouse's life that they don't want to address.
Parents who exclusively indulge one child are likely looking to these children to fill voids that these parents sense inside themselves. The mental health of these parents as well as their parenting skills are at risk.
It is probable that these dynamics will be reenacted in the subsequent generations of this family tree.”
^www.psychologytoday.com/us/blog/the-favorite-child/201104/when-favoritism-becomes-abuse^