I understand how you feel. I'm no exception to the rule when I say I'm territorial with my children. (they are grown and out of the house now but they are still my kids). I still want to protect and always will.
First, any conversation you have with her, you absolutely MUST have it with Trey in the room. No TV, no distractions, just you three. Keep Aiden with you and sitting between you and your husband. This re-inforces the bond you three have as a family and she can see that for herself.
Try not to attack her. She's obviously living in a place in her mind and in her heart that she's trying to show you. She loves the three of you dearly - she's just having a hard time letting go. Sounds like she might be so distracted by your family that she has forgotten to have a life of her own. Perhaps you could show her some new things to do in the community (church functions) or try to get her interested in some volunteer work at the neo-natal unit at the hospital.
The three of you need to do activities that take you away from the house a little more. Concerts, family outings at the park and things that just involve the 3 of you (yes, bring her along sometimes but not all the time). Show her that you can handle being a mom but try to do it in a way that will make a greater impression. Actions speak louder than words.
As for the will, I would never consider anything as important as a will left in just the hands of my husband. Not because I don't trust him (he's the best husband in the world) but because it's MY life. The belongings I love dearly go to the family member WE appoint as a mutual decision between my man and me so there's a clear understanding of how things will be.
You do not have to share your will with anyone but be aware that wills, for the most part, are public record once you file them with the court. Out of respect, give a copy to your children (if they are grown) or 1 member of your family who you consider emotionally mature enough to handle the contents of your will. Most of the time, the court will ask who the executor is that you've appointed. If you have a serious genealogist in the family, I would recommend appointing them if you consider they will be fair about things.
Aiden is your child. He belongs to you and Trey. Who takes over the role as caregiver is a decision between the two of you and no one else. Just remember, she's a mom, too.