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cf., Sent.I.D26.Q2.A2; QDePot.Q2.A5-6
Here, we go from asking about "real relation" to asking about "real distinction," i.e., whether the distinction between the relations in God are before the consideration of the mind or not.
St. Thomas' argument is simple and can be stated in two steps,
Major: There is real relation in God (as established in A 1, following upon the processions).
Minor: Real relation implies relative opposition.
ERGO, There is relative opposition in God
Major: There is relative opposition in God (as implied in the first syllogism)
Minor: Relative opposition implies real distinction.
ERGO, There is real distinction in God.
Thus, from this, we see that one must understand what this "relative opposition" is before one understands the argument (from it is the medium through which he demonstrates real distinction between the persons from the real relations).