Generally, even though you are legally married to your step-children's father, unless you adopted them too, your rights are not superior to the biological mother's rights with respect to them. Their father would have to be the one to seek to change the current custody arrangement. There may be some methodology to facilitate timesharing between the siblings. Consult with an attorney to discuss the specifics of your situation and your options.
Answered on Oct 13th, 2016 at 10:50 AM