Does KSA law permit me to marry my 16 year old stepdaughter?

I'm 54 yrs old and in Saudi Arabia. I would like to marry my 16 year old stepdaughter. Her age is not a problem but her relation to me could be a problem. I have been married to her mother for 3 years. I do not intend on divorcing her mother. So under KSA law, it is possible to be married to both of them at the same time or must I divorce her mother?