But I forgot my pen

Number one - and this is not specifically a female character problem - GRRM tends to avoid even mildly complex property questions in Westeros. No matter their rank, aristocratic families, even the greatest, tend to own a single seat, and it is notable when a House (like the Peakes) claims more than one castle, specifically more than one in the hands of the main ruling branch. (Don't even start with families or individuals holding seats across geopolitical borders.) I cannot entirely blame him in this decision - if you have even a passing familiarity with medieval England you understand how convoluted and arcane, especially to modern eyes, property law of that place and time can seem - but the consequence is that property disputes never get more complicated than, say, Tytos Blackwood surrendering some villages and mills to the newly pro-Lannister Brackens in ADWD. There are no manors or castles or lands for widows and ruling lords to haggle over because, at least as a matter of property law and inheritance, these holdings do not exist, certainly not in the author’s interest. Likewise, given the rather vague state of Westerosi jurisprudence, and the apparent lack of lawyers and law courts (other than my speculation on law-specialist maesters, which would not be quite the same thing anyway), we will likely never see the sort of legal property disputes that occurred IRL. 

Number two - and this is a specifically female character problem - GRRM has paid very little attention, certainly beyond the surface level, to the question of dowries. Indeed, GRRM does not even seem to know, and/or care, that there can be (albeit not always) a distinction between “dowry” and “dower”, much less that “dower” can (though again, not always) mean that provision given to a wife for her widowhood. GRRM almost always uses “dower” as simply a synonym for “dowry”, to mean what a bride’s family will give to the prospective husband and his family ahead of or at the time of the marriage being brokered. (I suppose you could argue that when Jaime remembers Cersei telling him that Tywin “had gone so far as to invite Lord Hoster to the city to discuss dower” when negotiation the Lysa marriage, Tywin and Hoster were preparing to discuss Lysa’s widow portion. However, even if that were the case, we certainly never hear about these discussions again.) Moreover, very few dowries we learn about in ASOIAF carry any sort of details, and certainly not including anything we might recognize as jointure. So “dowry” and “dower” for GRRM are not terms associated with widowhood and the rights/incomes of widows, but instead descriptors of prenuptial marriage arrangements.

Number three, I think GRRM only focuses on the above two topics when he has a plot point or story to tell with them. I understand that GRRM is an author primarily writing novels (and novellas) set in this universe; while TWOIAF and F&B exist, I would not call these GRRM’s first area of focus. I can acknowledge that while I, and probably at least some readers, would find discussions of jointure and widows’ property rights fascinating, I also understand that GRRM has a story, or stories, he wants to tell. So instead of, say, having Cersei and Margery squabble over royal manors, both are vying over being young King Tommen’s primary influence. Likewise, Myranda Royce, while still a very interesting character, is presented as less interested in enjoying whatever lands and/or manors might have come to her through her late husband and more keen on holding court for her father until and unless she marries again. Barbrey’s status as the Lady - or, as she wryly answers Theon, “widow of Barrowton”, emphasis Barbrey’s own - seems to be less the product of clear politico-dynastic calculus and more an expression of GRRM’s love for a smart, sassy, confident older widow character (especially in a portion of the story with fewer female characters overall). 

None of the above is an excuse, to be very clear.  I still find it utterly baffling and frustrating, for example, that Jocelyn Baratheon ceases to be a character (even more than the thinly detailed figure she was prior to Aemon’s death) after her husband’s assassination. F&B instead treats Jocelyn Baratheon as a walking womb, whose only importance in the narrative was to be born as Queen Alyssa died and then to give birth to Aemon’s daughter in turn, and she completely disappears almost as soon as Aemon dies. Too, the debate between Cersei and her father (and then Uncle Kevan) in her widowhood is not which (again, nonexistent) royal manor(s) Cersei would inherit but which prospective groom (like Oberyn Martell) would be her next husband and whether she would return to Casterly Rock. Indeed, even Rohanne Webber in "The Sworn Sword" focuses entirely on her position as Lady of Coldmoat in her own right, with no mention of any inheritance from her prior husbands - understandable in the context of a short(er) story about a plucky ruling lady in a patriarchal society, but frustrating in the context of this legal question. In other words, simply because we have not seen GRRM address these issues does not mean he could not have chosen, or could not choose in the future, to do so; every story he tells, or does not, is his choice to tell, or not. 

Now, you are correct to point out the general weirdness, in terms of narrative purpose as well as in-world application, of the Widow’s Law. I myself still do not have a clear idea as to why GRRM included the Widow’s Law in F&B, and given that we have not received any published material from the author since F&B, the importance or narrative use of the Widow’s Law remains an open question. The best answer I have come up with is that the Widow’s Law is GRRM’s version of the Salic law, specifically (it me) as depicted in The Accursed Kings - in other words, less a piece of property legislation than a flimsy (by authorial design) basis for politico-dynastic maneuvering. While it is certainly possibly future stories will use the Widow’s Law as a way to discuss jointure and widowhood incomes, I more anticipate that the Widow’s Law is going to come into play when, say, the various Stark descendants of “The She-Wolves of Winterfell” (or whatever its final title ends up being) start bickering over inheritance rights, or the Freys dynastically implode in the near future.