Is it "abstain from" or "abstain to"?

Solution 1:

In this instance, "abstaining" is an adjective - someone who currently abstains (specifically, from smoking).

We can rewrite the sentence like this:

Does the reaction of [people abstaining from smoking] to [the smell of other people's cigarettes] predict relapse?

So [people abstaining from smoking] react to [the smell of other people's cigarettes]

You "abstain" from something, but you "react" to something.

Solution 2:

Yes, abstain takes from, but that's not the way to parse this sentence.

This sentence is talking about the reaction of "abstaining smokers", and that reaction is their reaction to others' smoke.

You could say "Does the reaction of Andrew to the smell...", or rephrase the sentence as "Does abstaining smokers' reaction to..."

Solution 3:

REACTION TO (SOMETHING) Here, the noun 'reaction' takes the preposition 'to' after it.

ABSTAINING SMOKERS = PEOPLE ABSTAINING FROM SMOKING. Here, 'abstaining' is the present participle adjective, premodifying the noun "smoker".

So, the sentence is grammatically correct.