Ordinarily, you cannot use the other parent's new romantic interest to demand child custody modification. You can only use the new relationship to demand a modification if it threatens your child's welfare. Below are a few examples of such threats.
Drug abuse endangers your child's life both directly and indirectly. For example, the child might start experimenting with drugs if the new partner abuses drugs in their presence. In addition, drug abuse impairs judgment, which means the abuser might harm the child while the abuse is high. Thus, you may request child custody modification to safeguard your child from drug abuse.
You can also request a modification if the other parent's partner is abusive. That might be the case, for example, if the person is a convicted domestic abuser. A modification is also possible if you prove the person is an abuser, even without a conviction.
Consider a case where you learn that the custodial parent's new partner has been physically assaulting your child. You may get a custody modification to protect your child if you can prove the assault. Don't forget to inform the authorities about the assault since it is a crime.
The custodial parent's new relationship might affect your child indirectly. For example, the custodial parent might focus on their new relationship at the expense of the child's welfare. Some parents have even neglected their children in such cases.
Say the custodial parent does not provide the child with food, medical care, or education, and you can prove that all these started with the new relationship. You can explain these to the court and modify the custody arrangement to safeguard the child.
Courts make child custody decisions based on the child's best interest. A child's best interests do not always align with the child's wishes. A young child might not even know what is good or bad for them. For example, gifts can sway a young child and make them choose a parent who is not fit to care for them.
However, older children sometimes know what is good for them. The older a child is, the more likely the court will listen to their custody wishes. Thus, the court may consider a teenager's wishes if they do not wish to continue living with a custodial parent and the parent's new partner.
Both parents should look out for the child's best interests upon separation. Thus, you reserve the right to seek the court's intervention if you think the custody arrangement is not good for the child's welfare.
For more information, contact a local attorney.