The effects of favoring your own child over your mate's will have disastrous effects on your blended family. Favoritism is the kind of behavior that can tear a family apart. Not only will your stepchild be negatively effected, but so will your mate, your biological child, as well and your entire family unit.
The relationship between stepparent and stepchild is a delicate one. You find yourself in a situation where you have to be the parental figure, role model, and sometimes friend to a child that is not your own. Assuming that you have a decent relationship with your stepchild, it would do serious damage to your bond, the trust that the child has for you, and your credibility as a parent if you were favoring your own child over your stepchild. If you don't have a great relationship with your stepchild, not treating them equally will only widen the divide between you because you will have proved yourself unworthy of the child's trust and even respect.
Then there is the relationship with your mate. Just think about how much you love your child. Your spouse has no less love for his or her child. Now imagine that your spouse was favoring his or her child over yours. That would lead to some serious issues; wouldn't it? You would wonder if you married the right person. You would feel guilty for allowing your child to be subjected to that kind of treatment no matter what they did or did not do. You would feel the need to protect your child. Moreover, you would be confused that anyone who truly loved you could treat someone who is an extension of you so unfairly. These are the feelings that your mate would experience as a result of your favoritism. Needless to say, relationships with these issues don't last long.
Don't think for a second that your biological child is not paying attention either. Kids are a lot smarter than many adult give them credit for. Your little angel may pick up some pretty bad habits if they witness you favoring them over their step siblings. If you treat their step siblings like less than, they will too. If you decide to give them more attention, more praise, and more freedom they will take it. If you want to create a spoiled child, all you have to do is give them preferential treatment. If you want there to be tension between your child and their step siblings from jealousy, all you have to do is favor one child over the other. Lastly, you would be giving your child a false understanding of what love is. Love is not favoring your own child over your mate's. Love is kindness, respect, and boundaries.
In closing, favoring your own child leads to infighting between you and your mate, hostility toward you and your stepchild, tensions between all of the children, and an overall breakdown of the structure of your blended family. It is leads to chaos, hurt feelings, and feelings of entitlement. So if you ever find yourself thinking that favoring your child couldn't hurt that much, think again.