Lol about the 12 year old! But there are some situations where I think that would work for the family and not be strange at all. Just because she can give one outlying example doesn't make it the norm!
I cosleep with our 15 month old in a twin bed in her room. My husband can't handle the sleep disturbance, is a loud snorer, and doesn't share well lol. I'm ready for her to sleep alone whenever she is, but it won't be for a while I don't think.
If it is working for your family I don't see the problem. I don't think your therapist should be making blanket statements about cosleeping based on personal beliefs, but rather guide you in making the right decision for YOUR family.
I don't understand how it is creating "A lifetime of sleep issues" when most of us cosleep with our partners as adults, and a few generations ago sharing a bed with family (parents or siblings) was the norm and still is in many cultures.