Consider the spell true polymorph. If you turn a creature into a creature, the new creature
retains its alignment and personality.
If you turn an object into a creature
The creature is friendly to you and your companions.
An object typically has neither personality nor alignment, so that isn't normally an issue.
However, a sentient weapon is an object (per Sage Advice), and does have personality and alignment.
If you cast true polymorph to turn a sentient, neutral evil greatsword into a creature, does the creature retain the sword's alignment and personality?
Which could imply that it is not friendly.
Best Answer
The first sentence in the description of true polymorph says "Choose one creature or nonmagical object that you can see within range." Based on this, I don't think it's possible to cast true polymorph on a sentient weapon, as all sentient weapons are magical objects (as far as I know—I couldn't find anything that actually states this outright, but as V2Blast pointed out in a comment, the DMG covers item sentience in the section "Sentient Magic Items" starting on p. 214, and it appears to use the terms "sentient magic items" and "sentient items" interchangeably).