I posted about this in another thread- but I think it is extremely difficult to marry a man who was widowed after so many years with such a wonderful woman. The kids will probably resent whoever they end up with, no matter how nice they are. And being that woman is hard, because being a stepmother is an extremely difficult (and thankless) job most of the time.
Combine those stressers with the fact that Paul is probably one of the most famous/wealthy men in the world and I would imagine things would be very challenging. I have been in a stepmom role to two girls for some time, and have been involved in support groups for stepmoms for a while too. We discussed Heather's marriage to Paul when it happened and I think all of us were prepared to hear that the children he had with Linda supposedly hated Heather. If this is indeed true- then it isn't really Heather that they hate- it's the fact that Paul is with someone else and that person is not Linda, their Mother. I sincerely hope they can work things out, as I always felt from looking at them together that they had a great deal of affection for one another.