Learn Hardcore Latin: Misericordia sine iustitia interdum iniqua videtur. - Mercy without justice sometimes seems unfair.

It is nominative singular feminine, and that is why the rest of the sentence is built around it. In other words, the sentence is talking about mercy and then saying something about it.

A learner will often recognize this because:

misericordia is a 1st-declension noun
the ending -a here is nominative singular
it matches the adjective iniqua, which is also feminine singular nominative

Why is iustitia in the ablative case?

Because sine takes the ablative.

So:

sine = without
iustitia = ablative singular of iustitia

This is a very standard Latin construction:

sine amico = without a friend
sine cura = without care
sine iustitia = without justice

So the case is not chosen freely here; the preposition sine requires it.

What exactly is sine iustitia doing in the sentence?

It is a prepositional phrase modifying the idea of misericordia.

It tells you under what condition or in what respect the mercy is being considered: mercy without justice.

So the basic structure is:

misericordia = the subject
sine iustitia = a phrase attached to that subject
iniqua videtur = seems unjust