I don’t think anyone would think to criticise Jim’s relatives for not publicly congratulating his ex wife on her new pregnancy. Nic would be well within her rights to never speak of/to Tanya ever again, they haven’t been family for several years now.
So imo it does shed an interesting light on things, as Sarah (who really is just a mouthpiece for Jim) suggested that the marriage broke down purely due to Tanya’s infidelity. It’s hard to believe, if things were that cut and dry, that Jim’s own sister wouldn’t have fully cut ties with her.