This is just my opinion, but I think it is quite likely that Jason fell in love with Molly, the same way I believe the kids loved Molly. I don't think his family have ever denied that he loved her, in fact if you look at the comments on their wedding pictures etc they seemed to have a fairly positive, supportive relationship with each other, as normal as any other in-law scenario I have seen.
I think this comes down to experience. They watched Jason as a young man meet a woman and fall head over heels in love, they got married, built their own home, had their first child, set up their happy, happy lives together, enveloped in their wider extended family. Imagine the euphoria of that second pregnancy, Jack was to be a big brother, their family was expanding, it must have been such an exciting time...Sarah arrives and hurray, it's a girl - the perfect 2.4 family. Imagine the happiness and the joy that flowed through that house in those weeks. And then suddenly, out of nowhere, in one single night it's devastated. Mags is gone in an instant, no goodbyes, absolutely nothing. I think Jason will always have loved Mags, because there is no way to switch those feelings off. When you lose someone so instantly, you lose a part of yourself too. The Corbett's would have known that the Jason who continued on after losing Mags, was not the same Jason as before. I don't think it is a dispersion on any woman who would have come into Jason's life after this, but I don't think the family would ever have felt he could have loved her the same as the mother of his two children. IMO
That is not to say they didn't accept Molly as Jason's choice after the fact, it seems by all accounts that they supported the relationship (even if just for Jason's sake) in the years since the move to America. The fact of the matter is, Mags & Jason ARE reunited in Heaven looking over their two beautiful children, because Jason in now dead too. It really doesn't matter how much they accepted Molly in the intervening years, Jason died at her hand, I can understand they wouldn't exactly be singing her praises now!
Also, just a note on the life insurance money, it's been mentioned a few times (mostly on Molly's FB page) that the Corbett's are only after the money - this argument doesn't make any sense to me. They played no part in the murder, how does this equate to them being money grabbers?