((((((((hugs)))))))))))) It's a ill feeling to wonder if your parents are really your parents. To put that issue aside, I would ask for testing. Expensive, but maybe worth your peace of mind (and theirs too perhaps?)

No, no more swearing to secrecy. If they still have issues as adults in that relationship, that's their problem. You deal with this however you need to, and if it takes a family intervention, then so be it.

Maybe take them both with you to T???

I think for health issues, which with age will arise, you might also wish to know for sure.