Are 'resistant' and 'yielding' synonyms, antonyms, or neither?

These are antonyms!! 

Resistant means that something can't affect it (the resistant object), or has a hard time affecting it. For example, water resistant shoes aren't easily affected by water; water cant pass through them as easily as non-water resistant shoes.

Yielding means that something is flexible or is influenced by another thing easily. For example, when you "yield" when driving, you are letting the other cars go before you; or, when an object "yields" to pressure, it is bending under pressure (though not quite breaking).

In summary: resistant means nothing can touch it, while yielding means that things easily affect it; thus, antonyms.