none of the characters, including Marlo herself, were aware that she was suffering from such severe mental illness and therefore addressing it beforehand would have been impossible/wouldn't have made sense. I know that it's very important to treat depictions of mental illness carefully and with respect, but I see it as a non-issue in this case due to their lack of knowledge surrounding what Marlo was going through (and this is coming from someone who has battled severe anxiety and depression for years and continues the battle today, and is normally among the first to call out something that isn't right). It would have been more of a problem had Marlo/her husband/brother/sister-in-law/ etc known about her mental illness prior and it was still not addressed.