You are 100% right
@MrsDoyle, anger is a very negative emotion, which inflicts intolerable stress on your body.
Make love not war is my attitude to life and if you show nothing but kindness to OH's family, they can only react in one of two ways - stay hostile, which shows up their bad manners and bad attitude and will gradually alienate them from OH, OR they will melt and you can all share OH peaceably, which is what you want, plus you will gain from not carrying the stress of recalcitrant children.
Don't think about truce's, because that implies you think it will be a short-lived peace before hostilities resume; no one person can claim ownership of your OH and it seems very immature of his daughter to behave the way she is; carry on including her with her father's medical condition, but don't kowtow to her.
Best wishes and hang in there!