  The observed rotation measures (RMs) towards the galactic centre magnetar and
towards Sagittarius A* provide a strong constraint on MHD models of the
galactic centre accretion flow, probing distances from the black hole separated
by many orders of magnitude. We show, using 3D simulations of accretion via
magnetized stellar winds of the Wolf-Rayet stars orbiting the black hole, that
the large, time-variable RM observed for the pulsar PSR J1745-2900 can be
explained by magnetized wind-wind shocks of nearby stars in the clockwise
stellar disc. In the same simulation, both the total X-ray luminosity
integrated over 2-10$''$, the time variability of the magnetar's dispersion
measure, and the RM towards Sagittarius A* are consistent with observations. We
argue that (in order for the large RM of the pulsar to not be a priori
unlikely) the pulsar should be on an orbit that keeps it near the clockwise
disc of stars. We present a 2D RM map of the central 1/2 parsec of the galactic
centre that can be used to test our models. Our simulations predict that Sgr A*
is typically accreting a significantly ordered magnetic field that ultimately
could result in a strongly magnetized flow with flux threading the horizon at
$\sim$ 10$\%$ of the magnetically arrested limit.
