Thanks all for your valuable input :) Once again, I realize that the easiest path will be to just decide something, hehe. I should have known the real world is just as diverse as our games.
Higher status could indeed have an impact, I guess every house would want to see highly ranked persons as part of their House. About heritance, in my game all those older peerages requires male heirs, but I finally understood that the House and the Peerage is two different systems. So, even if peerages still run by their old fashioned patent, I assume the families (houses) need to follow the general community laws, being gender neutral.
Some suggested that Lady Grey would belong to both houses, and I might just decide that the sovereign of each House will decide who are members of his house. I guess only children who become criminal or upsetting their parents by marrying the "wrong" spouse might be declared no longer a member of House of Grey. Even that sounds a bit harsh, but it might be a logical consequence.
In the past I based my entire high society structure on those peerages, but it really messed things up when one house got 3 peerages of different rank. It makes the game interesting when I need to sort out stuff like this and not only have sims eat, sleep, work, knit and woohoo :)