First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
The problem that is described here is called a aterio-venous malformation (AV malformation, or AVM) , these are problems that people are born with and later in life they can bleed (causing a stroke) and/or can cause seizures. These lesions consist of arteries (blood vessels carrying blood away from the heart) and veins (blood vessels carrying blood to the heart) that are directly connected in an abnormal web of blood vessels. Normal blood vessels start at the heart as arteries and then become smaller arteries, then deliver the blood to your organs/muscles etc by capillaries (which are tiny hair like blood vessels). After the blood exits the capillaries it collects into veins that turn into larger veins and eventually reconnect to the heart. Arteries are under high pressure to deliver nutrients to the tissues, while viens are under low pressure. If they are directly connected (without any capillary network) this creates very fast and turbulent blood flow from the arteries into the veins. Along with the fast flow, the vessels are abnormal and tangled and often result in small bleeds over the years that go unnoticed, until a big bleed happens. These AVMs often have normal brain tissue that is caught up inbetween the web of abnormal blood vessels, making surgery difficult. There are different surgical approaches that can be taken to treat this AVM. One is surgical resection, another is selective embolization that is often combined with surgical resection. When AVMs are not causing problems (as in the previous 43 years of your brothers life) they can be observed and followed with imaging. However, when they are symptomatic (causing bleeds, stroke, seizures etc) then an intervention should be attempted to prevent further damage. I would suggest that you see a neurosurgeon at a major academic center for further treatment options.
I hope this has been helpful.