Jealousy is such a cruel mistress, darling...
Your posts sounds like 'my life is really crap and I hate every day of it, so I want them to ditch the stress free life that works for both of them, and also have a crap life'.
And you are invested in a life that's not your own. If you are raising these kinds of things with your brother where it's none of your business, don't be surprised if they become distant. You're literally meddling with their marriage in a strange and unnecessary manner motivated by envy.
Let's unpack a few of your points.
Why does she need to go out to the world of work? To make more money when they are already well-off? Why? Your next complaint will be that their cars and clothes are too nice and they take too many holidays, as they'll have more money than they know what to do with.
Nice houses unfortunately rarely come with less than 3 bedrooms. Me and ex had that same setup, I had a big office, he had a a large bedroom as a hobby room. I'm a single woman living in a 4-bed. Used to live in a 6-bed as that was the only property available of the kind I wanted at the time. Most of my single friends, or childless couples, live in 3-beds alone, smaller houses are really hard to find round here, unless you're willing to live in a rough area.
Then, does studying need to lead to a career? Have you ever done anything, especially acquiring knowledge, just because you're curious and want to challenge your brain? I have done several uni degrees just for fun, in fact, I'm about to do another, as I enjoy learning.
You seem to judge them for not having children, yet say they seem happy and at peace.
The only thing I agree with is 'full time dog mummy' sounding cringe, but hey, is your social media uber cool and all Kate Moss, with no #blessed on other cliches floating?
You are the sort of person that, if we worked together, I'd feel uncomfortable being myself around, smiling, breathing, thriving, being joyful, in the fear of being judged. Me and my best friend actually go and take our joy and laughter elsewhere at work sometimes, because we do have people around at work that judge things life yoga classes, creating a beautiful garden or investing in good foods and exercise for your pet.
The truth is, childless couples and childless single women, if they have an ok income, are going to have the 'easiest' life. No mess in the house, enough money left over even in average jobs, lots of time for oneself and hobbies, no tiredness, no backpain, no family drama. I could go on. Everyone makes life choices, it's their life, you need to learn to let them live their happy, blissful lives, and butt out.
Maybe just make your life something you actually want it to be. Start by not having a spare room full of junk? Who needs a room just for junk? Sounds like craziness to me.