I'd be okay with that, although it does pack a lot of characters in one movie when pulling from characters already confirmed to be in it would streamline things a bit. Either way, Sif should come back.
Seems like the Word of God is that Sif was exiled from Asgard before Ragnarok by Loki/Odin for getting to close to uncovering the switch and was Snapped and Blipped in Infinity War/Endgame. Plenty of time for her to be off doing her own thing and then come back.