I had thought it worked differently. A male inherits: therefore, Mary's son would inherit. I thought the concern was that she was not even engaged to anyone yet, so any opotential pregnancy would be some way off, and any potential sons would be even furtehr off. The family need to know who the heir will be, which is why they're trying to sort Mary's marriage out now.
The same principle applies in the Royal Family: Princess Anne has dropped down the list of heirs, past her younger brothers, because she's female.
Remembering my A-Level History, there was a Married Women's Property Act: I've googled it and it was 1882. This meant that a married woman's good and chattels no longer passed to her husband immediately on marriage. There were innumerable cases of men marrying rich women, dumping them, and then running off with the loot. Presumably this does not apply in the Grantham family because of Father's will?
I did find some of the material about inheritance ratehr tricky to follow. Mrs Chuff and I filled the ample commercial breaks by discussing it ...