actually u had a similar situation as my husband's sister-in-law when it happened to his 90-yr-old grandma all of a sudden. his sister-in-law was abt 5-month pregnant. she did show up with her husband (my husband's brother) and her older daughter (2-yr-old) and stayed probably for about an hour or so.
i guess most people didn't really mind if she should show up or not but eventually she did. i guess if she didn't, probably her own husband or mother-in-law might have something to talk about it later in case they're not in good mood or something.
after all, most importantly is how yr husband thinks and how your parents-in-law think. it might depend on how close yr husband and your parents-in-law are to yr grandma-in-law.