Robert wishes to avoid future trouble and complications about the succession.

That could be arranged by getting Sisily to sign some agreement renouncing all claim on the title." "I doubt if such a document would be legal, my dear," said her husband dubiously.
"That wouldn't matter in the least," replied Mrs.Pendleton, with a woman's contempt for the law.

"It would be purely a family arrangement.
Sisily could be assured by somebody in whom she has reliance--not her father, of course--that there was some legal reason why she could not succeed.

I do not think there would be any trouble with her.

She does not look the kind of girl to delight in a title and a lot of money.