"I am A's first partner and B has never had a girlfriend. They are 29 and I am 26."
And you've been together for four years, so got together when you were 22 and he was 25. 25 year-old twin brothers buying a house together. That's quite a financial commitment to each other. Even if they make the case that they had to get on the property ladder ASAP, it makes it clear that they expected to be living together for several years at least - they would lose money (on fees, stamp duty etc.) otherwise. How long do you think they anticipated living together?
"Living here was supposed to be temporary to save money but it feels like there is no end in sight and I have no idea what to do."
It feels that way because it is that way. There is no end in sight.
"I love him so much and want to make this work but have no idea how to tell him how I feel."
Why do you love him? I'm not seeing anything particularly loveable in his behaviour. He shows very little consideration of you, living essentially a bachelor's life with his twin, and having you on tap for sex. If you question him
"He is very defensive of his twin and I can't speak to him about these things without him getting defensive and often angry. He shuts down and stonewalls me." Not very loving towards the woman who moved into an unsuitable home to be with him. And this is "the more sensitive brother"? Really?
You say of his brother that perhaps "because of him being nearly 30 and never dated, I've heard him express some views on women and relationships that I would label a little misogynistic. This adds to my discomfort somewhat." I should think it does add to your discomfort. After all, you also say of these brothers that they "constantly reinforce each other's opinions about everything". You know that what B expresses, A thinks. Don't you?
"They have a very insular friend group. One of their close friends only getting a girlfriend this year at the age of 30."
There is an old saying, that 'a man is known by the company that he keeps'. Look at his friends and his brother, they are an unfiltered version of him. He'll tone it down to your face, but - birds of a feather flock together. And this particular flock sound like a bunch of incels. Women are the great unknown, hence the misogyny and lack of relationships. And just because your boyfriend isn't actually celibate (he has you) doesn't mean he's lost the mindset that comes with it.
"I love him deeply and he is 100% the man I want to marry one day."
Again, why do you love him? Because I am really wondering if you love an idea of him rather than the actuality. He managed to impress the 22 year-old you to such an extent that you moved to a deeply unsuitable house (suburbs etc.) just to be with him. You put up with the deeply unsuitable living arrangement of his brother being there. You continue to put up with the deeply unsuitable being treated as an afterthought whilst he lives the bachelor life with his "very insular friend group".
I suspect you have fallen prey to the Sunk Costs Fallacy, whereby, having invested so much of yourself into this relationship, you are unwilling to let it end because then, you would have 'wasted' the past four years. And so you suppress your doubts and your discontent and talk of "the man I want to marry one day". Personally, I think you need to give yourself a shake and look at your reality. You are not his priority. You're not even a close second to his twin, you are trailing far behind his incel friends and his sport three times a week and his video gaming late into the night. You are an afterthought.
So, you asked for advice. You won't like mine, but I hope you take it.
Leave this relationship. You're only 26, there's a big wide world out there and it is your oyster. Don't consider the last four years to be wasted but do consider them to be a learning experience from which it is time to move on. Do not throw good money after bad, in your case do not spend more years on a relationship that will never deliver anything more than it does right now. In this relationship, you will always play second fiddle to his twin, his friends, his sports, his video console. Moving on, you will find a respectful partner who will make you feel loved and cherished and his priority, and you will be happy. Leave this relationship and move on. It's time. ((hug))