It seems pretty clear that Cora's brother, Harold Levinson and Lady Rosamond are both childless. Whilst this could still possibly change for Harold I think Rosamond is definitely past child-bearing age.
If Marmaduke Painswick and his family are relatively self-made then we can assume that Rosamond's wealth/house in Eaton Square is her own and not merely just a life interest which will pass to one of her husband's relatives when she dies. So when Rosamond does die, wouldn't that mean that she is most likely to leave all of her wealth to Mary, Edith and Sybbie (as Sybil Snr's only daughter). So if Tom does end up moving to Boston, like it or not it seems that young Sybil is still going to inherit some serious money and be a very wealthy, upper-class lady.
Furthermore, Cora's brother Harold is getting on in years and he may not have children if he continues he womanising ways (although I would like to see him marry Madeleine Alsop). I think in Season 3 Mary said there was only 'mama and uncle Harold' to share the vast wealth of the Levinsons, and so upon the deaths of Martha and Harold Levinson and Harold is wifeless/childless wouldn't Cora or her children inherit the huge fortune of the Levinsons?
All this of course is assuming that they all don't go bankrupt in the Wall Street Crash of '29.
What do you think?