This makes me think of my family. My mother is very narcissistic and my sister can have many traits. You need something you can call your own, your own therapist and space. It sound to me like a boundaries violation on your mother's part. I often feel guilt whenever I put my needs first and especially when it comes to siblings. This is because I was so often guilted as a child and often treated as the scapegoat. It feels good now (though very nerve wracking) to designate my boundaries to both my therapist and family. I do not think you are over reacting at all, and your feelings of discomfort regarding this is a healthy sign of your boundaries. Definitely talk to your therapist about this even if you feel guilt. I love the DBT skill in response to guilt that is not justified- continue to do that which makes you feel guilty. So, in other words, you do not deserve these feelings of guilt you are totally in the right to have your own separate therapist and to have clear boundaries with your family. So continue to hold firm to this need in the face of guilt and the guilt will subside.