I don’t like how friendly my husband is with my sister and how friendly she is with him. Maybe it’s just me being insecure but I get the feeling he wishes he’d met my sister before he met and married me. I think he thinks she’s a better match for him than me and I think my sister thinks my husband wants her. She flirts with him right in my face and acts like they’re just having some harmless fun and tells me I’m getting offended for no reason.
Talk to your husband about it. Tell him how you feel. Admit that it makes you feel like he’s more interested in your sister than he is in you. If your husband cares about your feelings he will not only listen to you he will also try to be more sensitive to your feelings when he’s interacting with your sister. If he refuses to make any adjustments in how he interacts with her he’s being totally inconsiderate of your feelings. Whether or not you have reason to feel insecure is not the issue. The fact is you do feel insecure and a man who loves his wife will not prefer to enjoy some “harmless fun” with his wife’s sister than to be a little bit more guarded so his wife can feel more secure in his love.
As for your sister, if it’s possible to limit her opportunities to interact with your husband then do it. She’s causing problems in your marriage. If she was a thoughtful and considerate sister she wouldn’t flirt with your husband and dismiss your unhappy reaction with accusations that you’re getting offended for no reason.