  An important requirement for spin-based quantum information processing is
reliable and fast readout of electron spin states, allowing for feedback and
error correction. However, common readout techniques often require additional
gate structures hindering device scaling or impose stringent constraints on the
tuning configuration of the sensed quantum dots. Here, we operate an in-line
charge sensor within a triple quantum dot, where one of the dots is coupled to
a microwave cavity and used to readout the charge states of the other two dots.
Owing to the proximity of the charge sensor, we observe a near-digital sensor
response with a power signal-to-noise ratio >450 at an integration time of
$t_{\rm int}$ = 1 $\mu$s. Despite small singlet-triplet splittings $\approx$40
$\mu$eV, we further utilize the sensor to measure the spin relaxation time of a
singlet-triplet qubit, achieving an average single-shot spin readout fidelity
>99%. Our approach enables high-fidelity spin readout, combining minimal device
overhead with flexible qubit operation in semiconductor quantum devices.
