Their parents should never have disowned Sirius. They should never have demanded he join the Dark Lord's forces. If they had known their son at all, they would have recognized that he wouldn't agree even under the Cruciatus. Regulus still wondered whether they did actually know, and did it on purpose so they would have some excuse to disown him. Perhaps Regulus would never know for sure. He wasn't such a fool that he would tell anyone or, Merlin forbid, ask them.