Is it necessary for someone to obey his parents ( stepfather and stepmother)?
You should obey your stepparents otherwise it would lead to conflict. However it is not obligatory. Obedience is only obligatory towards biological parents. Regarding the rights between stepparents and stepchildren, the scholars at Islam-QA say:
But spending on maintenance, serving and obeying are not obligatory between them.
It may become obligatory if your real parents tell you to obey your stepparents.