From Don Juan:
This heathenish cross restored the breed again,
Ruin'd its blood, but much improved its flesh;
For from a root the ugliest in Old Spain
Sprung up a branch as beautiful as fresh;
The sons no more were short, the daughters plain:
But there's a rumour which I fain would hush,
'T is said that Donna Julia's grandmamma
Produced her Don more heirs at love than law.
Does this imply that she had extramarital relations, and these heirs had other fathers?
The last paragraph of the question understands the line correctly: according to the rumour, more of Julia’s uncles and aunts were illegitimate (‘heirs at love’) than legitimate (‘heirs at law’). The OED says:
heir-at-law n. the person who succeeds another by right of blood in the enjoyment of his property
This phrase was popularized by the comedy The Heir at Law by George Colman which opened in London in 1797 and would have been familiar to Byron and his readers in 1819 when the first part of Don Juan was published.
The phrase ‘heir at love’ is parallel to ‘heir at law’, but this is ironic because if a child were known to be illegitimate, they would be no heir at all.
