I'm Not His Real Mom

So my step son is 15 and has autism. U have been un this boys life since he was 6. His mom dropped him off at his dad's one day and never came back. He went to court and got full custody and decision making. She will call maybe three times a year and that's me being nice. My husband knows I'm this boys mom and let's me make decisions like I'm his biological mother. So she called this morning and asked if he could come to her house. There was this event at a school thats supposed to support parents of special needs children and give them resources or something. She was hoping he could come for a few days for the event. I flat out said no. She lives in a completely different state and hasn't seen him in 12 years. This was also her first time calling in 8 months and it's so she can flaunt him at an event. Also she doesn't have custody of him so what resources she needs for him is beyond me. She then says I'm not his real mom so I have no say. That pissex me off. I've been there nearly 10 years. Every graduation. Every birthday, I was the one there. She talked to my husband and he also said no. She didn't even ask to speak to her son. The audacity of this lady to say I'm not his real mom. I may not be his biological mom, but I'm his real mother