I do not think he was willing to put at risk his sister's character, I think he confided in Elizabeth's secrecy, but I think like you it was showing his love, and he aknowledged his motive :
"Here again I shall give you pain—to what degree you only can tell. But whatever may be the sentiments which Mr. Wickham has created, a suspicion of their nature shall not prevent me from unfolding his real character. It adds even another motive."
The motive of protecting her of course : if he had another motive (his love in spite of her rejection) he could not avow it, given what he wrote at the beginning of his letter.