Hello @rcb1975, I'd like to add my welcome to Mayo Clinic Connect.
I am sorry for the losses you have experienced. It certainly is true that one loss can bring up the grief from a previous loss. The unexpected loss of your beloved cat might have brought up feelings of grief from the unexpected loss of your brother.
I wonder as Colleen (@colleenyoung) does if you have attended grief counseling workshops, grief groups or one-on-one grief counseling. The unexpected loss of your brother when you were a preteen is very difficult. I have a family member who at age 16, lost two brothers in a tragic accident. Her brothers were preteens at the time and the family just never recovered from it.
There are grief groups for the loss of family members. Even though it has been 30 years since you lost your brother, a grief group to attend regarding that loss might still be very helpful. Right now, most grief groups are all online (using Zoom, Facebook, or another online method). I would encourage you to look into this and find something that might be helpful to you right now.
Also, I would encourage you to keep a journal. Even just writing a sentence or two each day about your brother, then about your beloved cat, might be very helpful to you. Will you give these ideas a try?