Fraternal grandma and 2nd cousin died of brain aneurysms at ages 38 and 42. I am 33 and had a clear mra at 27. Do I need repeat screening and if so how often?

Aneurysm. I am not sure what you mean by 'fraternal grandma'. That said, if an mra with contrast with clear visualization of the entire circle of willis was normal at 27, and was reviewed by a neuroradiologist, then you might consider a repeat study in three to five years to be surer.
Perhaps one. 'fraternal'? That's brother; , likely either maternal (mother side) or paternal. Which side is second cousin on? Either way, a magnetic resonant imaging study is the correct choice. I think it would make sense to do an mri/mra now by a well trained neuro radiologist that knows your exact history. Also have a neurologist get a baseline hx and px as baseline, should any clinical sx develop.
Bare minimum. As long as the mra was of good quality and reviewed by someone who is experienced in neuroradiology. If it is in fact normal, given your two relatives with fatal aneurysm ruptures, i think screening every 5-10 years is reasonable until age 60. Average age of rupture is 52. Please refer to brain aneurysm center for women for more information.