Cascade is a triggered ability that triggers when you cast a spell. The ability will go onto the stack above the spell.
Assuming Possibility Storm is the first spell you cast this turn:
Because Possibility Storm is still a spell, its ability will not be relevant. When the cascade ability from Maelstrom Nexus resolves, you'll cascade for a nonland card with converted mana cost 4 or less, and you may cast that card without paying its mana cost.
Once Possibility Storm resolves, the first spell you cast from your hand on each turn will trigger two abilities: cascade and Possibility Storm's ability. The abilities are independent of one another. You choose the order in which they are placed onto the stack. Both abilities will resolve normally, regardless of the order in which you resolve them.
Note that Maelstrom Nexus only gives cascade to the first spell you cast each turn, and Possibility Storm only triggers for spells you cast from your hand. Possibility Storm's ability won't apply to cascaded spells, and Maelstrom Nexus's ability won't apply to Possibility Stormed spells.