In I Peter 5:13 Peter said "The church that is at Babylon, elected together with you, saluteth you; and so doth Marcus my son." Since Peter was married, could Marcus be his son?
It wasn't unusual for preachers who worked closely with younger men, who would eventually succeed them, to refer to them as their sons. Paul converted a runaway slave, whom he refers to as his son. "I appeal to you for my son Onesimus, whom I have begotten while in my chains" (Philemon 10). Paul also calls Timothy his son. "You therefore, my son, be strong in the grace that is in Christ Jesus" (II Timothy 2:1).
It wasn't restricted to men either, Paul referred to Christians in congregations where he worked as his children. "My little children, for whom I labor in birth again until Christ is formed in you" (Galatians 4:19). John did the same. "I have no greater joy than to hear that my children walk in truth" (III John 4).
Thus we conclude that if Peter was referring to John Mark, which seems to be the case, then it was a term of endearment. We do know that Barnabas was John Mark's cousin (Colossians 4:10). We also know that John Mark's mother was one of the Marys mentioned in the New Testament (Acts 12:12). Peter and Mary were not married because in the incident recorded in Acts 12, when Peter was released from prison, it says he went to Mary's house and not his own home. Since it is referred to as Mary's house, then it is likely that Mary was a widow.