Jattitude
As I pointed out in Friday's update thread - if Kamala wants a mother for Vihaan, why doesn't she just ask Vani to be his nanny, instead of going all the way & marrying Murli? After all, she's seen sauteli behavior in her family itself from Gau and Ashtami. Why does she automatically assume that if Vani has her own baby, she'll treat it equally w/ Vihaan, and not give him step-motherly treatment? She doesn't once stop to think about whether Vani would actually be a suitable mother for Vihaan under all circumstances?
Vani's best response would be to tell Kamala that she can by no means promise that she'll be fair to Vihaan should she have a child of her own. This should stop Kamala in her tracks. And Gai - previously, she did everything to sabotage Sudha's marriage to Murli, including inviting RA to wreck the Holi celebrations, and now, she is doing everything to encourage Murli's re-marriage. How does that help her, when she knows that Bharti is alive?
The example you gave only works if the woman in question doesn't plan to have more kids. Otherwise, let's say she has kids from her 2nd husband, is it fair to her current in-laws to have their new grandkids stay as well w/ her ex-in-laws? I would think her current in-laws would understandably be resentful of such an arrangement, even if they are broad minded enough to let their son be not just a ghar-damaat, but on top of that, live in his new wife's ex-sasural. It's one thing that a son moves away, but less acceptable that their grandkids call someone else, instead of them, their Dada & Dadi (which they probably would, following the example of their half-bro/sis)