Am I wrong for feeling irritated by my step brothers relationship?

My(22m) girlfriend(22f) and I have been together for 5 years and we have a really solid relationship. We had a honeymoon phase but it faded after 6 months. My step brother(22m) and his girlfriend(27f) have been together nearly 4 years and their relationship seems like its still in the honeymoon phase.

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My step brother is sure that she's "the one" he's said before that he's not sure when but he's gonna marry her. That's great for him but I'm kind of confused as to how he's sure he's ready for marriage yet. You'd think since im older I'd be ready for marriage first but I'm not where near ready for marriage. They're so content and spontaneous with eachother. I checked her Instagram story today and apparently they're at the beach because it's sunny and Friday. No prior plans or anything, just because its Friday and sunny. It's irritating because I feel like I'm behind because I'm not ready for marriage and i don't know where to even start on being spontaneous. She seems so happy with him because they're spontaneous and madly in love yet I don't have that excitement. My girlfriend hasn't said anything but she keeps saying that they're a cute couple and its so irritating. Am I wrong for feeling this way?