If it can't produce offspring, is it considered incest
?
Incest is any sexual and or romantic relationship with a relative (generally close) whether biologically related or not.
Uhm, not to rain on your parade, but if they aren't blood-related it's Pseudo-incest
. Very different.
Tolerably sure most laws and social/cultural norms do not recognise any meaningful difference there.
Where do you live? Most laws actually do make a difference there! For example, if your parents remarried and you got into a relationship with your step-sibling that does not count as legitimate incest and has no law related repercussion in most countries.
Now socially or culturally, I can definitely understand the issue. It certainly has a stigma by default, but it's just not the same. When Pseudo-incest is bad it's usually for different reasons (like age gap, abuse, predatory tendencies etc.).