Help...Stepson asked if he could call me Mom...!?

Okay, so I'm married to the most wonderful man ever. We have four beautiful children together and live a very happy life. He was married previously before we met, and had two children with his first wife, Lily. Lily has never cared for me, and I just always get this horrible vibe from her. My step children, Ashley and Mark, come to our house every other weekend and I love them both as much as my own children! Mark came up to me on Saturday and asked if he could call me "Mom" instead of Clare. I didnt know what to say, so I told him he could call me whatever he wants...I'm uneasy about this because of his MOM Lily...who is always trying to start something...call it "baby mama drama" if you will. What should I do? I asked my husband and he said it was Mark's decision on what he called me, and I've been like a second mother to him since he was 2 years old. Okay...thanks for any help.