If you're a step parent you don't have the legal right to make choices about a stepchild. It doesn't matter if you're married to the child's parent or not.
You can get the legal right to make choices about a stepchild if you're given parental responsibilities and rights by a court.
You should talk to a solicitor if you want to apply to a court for parental responsibilities and rights.
If you're splitting up with your partner and can't agree
If you can't agree about what to do, you can get help from a family mediator.