I think it's interesting to compare the use of the past participle "left" in sentences like the one in question with the past participle "parked" in similar sentences:
1a) Those are two only two relatives he has left in his family.
1b) He has only those two relatives left in his family.
1c) Only those two relatives are left in his family.
2a) Those are the only two cars we have parked in our parking lot.
2b) We have only those two cars parked in our parking lot.
2c) Only those two cars are parked in our parking lot.
In each case, the past participle conveys the sense of "remaining" (indeed, "parked" could be replaced with "left" in 2a, 2b, and 2c), and yet each past participle has its own meaning. The only difference is that we can clearly postulate a "parker" in the second example set; the vehicles came to be in a state of being parked in the parking lot by virtue of having been parked there by someone. We don't know who parked them there, but we know that someone must have.
In contrast, there is no clear "leaver" in sentences like those in (1). At most, we can say that it is as if the relatives were left by destiny, fate, God, the relatives who "departed," etc. In neither case is the speaker really thinking about an agent, or even about the act related to the past participle; yet we can't say that the past participle is devoid of verbal meaning. There is something of the stative/quasi/generic-subject passive voice in each case—in my humble opinion.