A user made a Python library to help handling the .sif file:
I didn’t test it though.
SIF file format is described here: Synfig File Format (.sif) — Synfig developers docs documentation and here Sif Format - Synfig Animation Studio (old).
If you need examples, Veer Metri made some plugins, posted here on forums and keep them in its Github account: veermetri05 (Veer Metri) · GitHub