Well, that's very family friendly.
OP actually cites a reliable source stating that song should be playable in Lego Rock Band: Alive (Live: Drop in the Park), which lists the song as "family friendly". Unless there is some other requirements I don't know, that qualification is the only thing necessary for DLC to appear in Lego Rock Band.
Who cares? The simple fact that you managed to buy it shows you have RB2/1.
Witticus: "GeeNef speaks to me like schizophrenia, his words touch me where my priest could reach."
Yeah...if you want to make sure that songs are playable in LEGO...buy them through the LEGO disc or go through the LEGO Rock Band listing in the PSN/XBLM.
Tiger Uppercut: Platinum Artist | Expert: G/B | Hard: D/V
Song Count: 831 | Newest: "Great Balls of Fire" & "Dog Days Are Over"
Also, the fact that (s)he managed to buy doesn't mean anything. Xbox Live Marketplace page for Rock Band 2. You can download any song/trackpack from there without owning the game (useless, but the same as you remark).
Why would you buy content for a game you don't own? If you know that Lego Rock Band supports DLC, you probably also know that there's a filter and that you're better off looking in the Lego Rock Band section of the Marketplace.
Truthfully, I never knew there WERE incest overtones in the song. I always took it more as just someone being a horrible mom in another way (Oh, by the way, that dude you thought was your real dad isn't, your real one died awhile back, thought you should know).
It really just adds another level of disturbing to it, really.
Clutch now in RBN! Go download!