Webster's 3rd unabridged shows fewer than four columns of in-n* words, and fewer than five of those words are negative, innumerable being the most familiar. For most, the in supplies the sense of รขโ‚ฌยฆ The preposition to can express a possessive relationship.

The key that belongs to or belongs with this lock. In your sentence, to works well because couples are entities, รขโ‚ฌยฆ