Is it a big deal I hang out with my girlfriend's sister?

Anonymous
I'm 27, girlfriend's sister is 26, my girlfriend is 23.

I've known my girlfriend's sister longer than I've known her, and both of them are pretty, but her sister is a really good, supportive, loyal friend.

We never dated. In fact, we always were with other people, but we always looked out for & took care of each other.

My girlfriend and I have been together 9 months, and I've never given her any reason to believe I'd lie. I've always come through for her when I promised & put her first.

She gets jealous cuz she thinks I want to ditch her to be with her sister.

I told her both of us have insecurities, but her sister was my friend before I knew her, so she needs to get over it.

Like, a couple nights ago, I was home sleeping, and woke up to find texts, "I know you're with my sister" "fuck you both" and other stuff.

Her sister is very nice & supportive, goes out of her way to make it clear we're just friends, and tries to help me deal with my girlfriend's insecurities.

I've never told my girlfriend no when I was free, or put her sister ahead of her, but I'm not stopping being friends with her just because we started dating & she needs to confront her insecurities.

Her sister has a boyfriend, but when my girlfriend is busy at work or out with other people or whatever, sometimes her sister and I will go grab lunch, or do stuff when both of us have nothing else to do.

She hangs out with & talks to other guys, and I told her like any other girl she needs to either trust me or break up if it makes her that uncomfortable, cuz if she can't' trust me then it's never gonna work between us.
Updates
+1 y
Also I consider her sister isn't going anywhere. If I'm with her both are always going to be part of my life, so trusting me isn't an option. If our relationship is going to work she's going to need to or break up.
Is it a big deal I hang out with my girlfriend's sister?
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