Should I see my sister wanting to go to the Brooklyn Labor Day parade with my husband as a red flag?

I just recently found out that the reason why my sister started getting upset with my husband and turned my mother against him was because she asked my husband if she could go to the Brooklyn Labor Day Parade with him and his friend while they sold nutcrackers. He told her that he’ll stay home and she could go with his friend to sell them. She got upset because she wanted him to come along. My husband didn’t want to go because he knew that she would whine on him and he didn’t want any parts of that.
She has since started a smear campaign against him.
I find it very strange that she would be so angry about him not wanting to go with her. I also find it strange for her to ask to go with him in the first place. Am I wrong for thinking this?
Should I see my sister wanting to go to the Brooklyn Labor Day parade with my husband as a red flag?
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