To my knowledge, there is no sharing of benefits for the brands under CCL Corp, only the ability to obtain past passenger discounts. At the time that Princess was owned by P&O, those were interchangeable, and when Princess bought Sitmar, those benefits did transfer. But if you've for instance sailed on Cunard, or HAL, those benefits do not transfer over when sailing on Princess.
To my knowledge, although I could be wrong, Home Lines and Royal Viking were never part of CC Corp.
Also, if one person in a cabin is of a higher level than the other, the person with the lower level does not automatically get bumped up. I for instance am Elite with Princess and I get the black card, but Frank is only Platinum so he gets the platinum card. Now someone sailing in a cabin with say an Elite member, would get to take advantage of most benefits, such as early boarding, laundry,etc, and probably be included in any "special invitations" that might result - but the internet benefit for instance would not apply unless both parties were using the same account, which Frank and I don't, we both set up our own accounts, so unless there is a platinum internet benefit, he would not get that, but I would get the Elite benefit for internet.
The same holds true with Royal Caribbean, Celebrity and Azamara, the benefits on one do not apply to the other, with the exception of getting the past passenger booking discount.