Overall this is such a mess that it's a case where having a competent immigration lawyer would be worth it.
And note: a mess that's almost entirely the fault of all concerned in the family. If the OP had simply submitted a PRTD application immediately on becoming 18 years old, when argument that was a minor before that, this would long ago have been resolved positively (likely anyway, nothing 100% here). And if they'd done a simple lawyer consultation, they could have done it all themselves afterwards i.e. cost shouldn't have been prohibitive.
Now? Well, good luck.
Or Canada could also stop allowing what appears to have happened here. Family got PR and briefly moved to Canada decades ago. Have used the PR as a vehicle for children to immigrate which often involves attending post secondary education at domestic rates.