OED [subscriber-only link] has law as a verb:
2.a. intr. To go to law, litigate. Also to law it. Also colloq. or dial. in indirect passive.
?a1550 Hye Way to Spyttel Ho. 799 in W. C. Hazlitt Remains Early Pop. Poetry Eng. IV. 59 They that lawe for a debt vntrew.
1866 ‘G. Eliot’ Felix Holt I. Introd. 13 People who inherited estates that were lawed about.
b. trans. To go to law with, proceed against in the courts.
1647 J. Trapp Comm. Epist. & Rev. (1 Cor. vi. 7) By your litigious lawing one another, you betray a great deal of weaknesse.
1870 E. Peacock Ralf Skirlaugh II. 117 You can't law a man ye knaw for a job like that.
Calpurnia's use of lawing appears to have this meaning, or a meaning very closely related. The family is a family involved in the law and litigation, and lawyers are renowned for making use of any nuances of meaning they find advantageous — meaning that very little is entirely clear-cut in any argument.
That entry was first published in 1902 and doesn't appear to have had a great deal of attention since then, so if the use of law as a verb fell out of use around that time then it wouldn't be recorded as obsolescent. Smaller dictionaries which have been updated since then might not list the use at all if it turns up so rarely that it's no longer worth including.
However, an obsolescent use can often survive in local dialects.