My fiance drops everything, rushes to his sister during any of her many ‘emergencies’

DEAR ABBY: I am marrying the most amazing man next year. We have been together for six years. I have always gotten along with his family and know they will be great in-laws. However, as time has gone by, I’ve noticed that they cater more and more to my fiance’s older sister. She’s nearly 30, but she acts like she’s 13.
When she has an argument with her spouse, she calls my fiance and makes it sound so dramatic that he drops everything and runs to her rescue. Invariably, it turns out to not be as bad as she was saying. She also talks so badly about her spouse to her parents that I feel embarrassed for him. If she’s sick or home alone (her husband travels for work), she sleeps at her parents’ house with her infant son instead of staying in her own home, which is a town away.