Lowest possible latency, 1-way RF transmission
Your problem isn't solvable by lower transport latency. That's simply unrelated to your problem; your transport latency might be arbitrarily low, but with a random offset between your controller initiating sampling and it sending the data, no direct conclusions on the start of sampling might be made.

Conversely, you could just as well have days of latency, if just all sensor data had precise timestamps, so that you could align the data in the PC.

Now, your 1ms synchronity is really not that much of a challenge with modern electronics. Your MCU will have no hard time at all triggering anything with a timing ambiguity far smaller than that.

A usual approach here is to share a common time source – there's a lot of options here; the established ones are


A shared trigger signal. For your bandwidths, a simple pulse over twisted pair or coax to all sensors so that they start converting synchronously. In measurement equipment, you'll often find PPS in- and outputs.
A shared, dedicated trigger RF signal. To achieve a 1ms accuracy, your pulse's bandwidth should be >> 2 kHz (which really is nearly nothing). There's plenty of GMSK / FSK modems that would achieve that, as a side effect of being able to receive packetized data. As Jeroen3 pointed out, you could also just build an FM modulator and a matching detector (in discrete electronics!).
Same goes for optical communication: Transmit a pseudorandom sequence of 0s and 1s with one microcontroller using a strong IR LED, and let all sensor nodes look for that sequence in their MCU code. Use cross-correlation to find the temporal "center" of the sequence. Synchronously start sampling a fixed interval later
GPS receivers often have a PPS (pulse per second) output. That's what is used to synchronize a lot more timing-sensitive systems (e.g. cellular base stations, which need way better synchronicity than +-1ms)
Abuse some easily receivable RF signal to find your timing – for example, let the sensor also record a bit of DVB-T spectrum for a short duration. On your PC, record the DVB-T spectrum continously. Then, cross-correlate sensor-observed and PC-observed RF signal, calculate exact start of sampling at sensor. Again, your observed bandwidth limits the accuracy; the higher the bandwidth, the lesser the timing ambiguity. For your 1ms case, however, as mentioned, a couple kHz would do, and FM Radio would also totally do (just that in this case all your devices only are receivers and you don't have to worry about not messing with FCC/BNetzA/OFCOM regulations).