Congrats peachyyyy_ sa 189 invitation!
Regarding your question - same kayo ng sister ko. Yes, puwede kayong magpakasal AFTER lodgement but BEFORE visa grant — and it will NOT negatively affect your 189 application. Since kasama mo na ang de facto partner mo as secondary applicant, the marriage will not reset, cancel, or delay your application in any major way.
What you can is, I-notify ang Department na nagbago ang civil status mo (Form 1022 – Change in Circumstances), then I-upload ang marriage certificate kapag available
Optional but very helpful daw sabi ni cousin: updated relationship evidence (photos, joint docs, etc.)
Walang naman naging issue sa visa nila.