Pretty much anyone who has entered the arena of blended families and step-parenting knows that it is like trying to walk across a field of landmines. Typically, the biggest problem is that the step-parent thinks the step-child should love and respect them just like they would their biological parent. Unfortunately, it doesn’t really work that way.
Here’s the best piece of advice I ever heard on this subject. As the step-parent, take the time (perhaps years!) to sow nothing but love into the relationship with your new stepchild. Resist the desire to step in as disciplinarian and let the biological parent handle that. As a couple, talk with your spouse behind closed doors about how “situations” and discipline should be handled, but then let the biological parent actually carry it out.
The idea of sowing love into a person before expecting that person to love you in return is actually demonstrated by Jesus. The Bible says “We love because he first loved us” (1 John 4:19). We need to model this as step-parents. Your step-child needs to see you demonstrate love for a sustained period of time before believing you actually have their best interests at heart. This may truly take years, especially if the child is over the age of about 5 when you first enter the new marriage. Be patient.