Being an uncle myself, I felt awkward enough buying my own niece 'Bravissimo' vouchers for her mid/late twenties birthday at her own request, so reading this post struck a (thankfully dissimilar yet unnerving) chord...
First off, at face value this event is notably abnormal and needs addressing one way or another. Without the faintest trace of doubt. Not just because of the singular event in question but because, by your own admission, this isn't just a one-off 'crazy jape'; your brother-in-law has acted inappropriately in the past. Whatever lies behind your statement, this is REALLY fucking important! At the very least, please do not lose sight of this.
A couple of initial questions (for your own mind at least, I'm not asking you to answer them openly):
- Any idea what caused him to send this totally unwelcome gift to your DD in the first place? Have you asked your DD what sort of previous dialogue they have had in order to arrive at this point?
I expect many will say it's entirely irrelevant. He was 'grooming her' etc, - and myabe he was. In no way does it justify the ultimate act but understanding a bit more about her take on matters (you haven't provided any prior insight; not that you should...) might inform your approach to dealing with it by putting the whole thing in a bit of context; without which you're just left flailing in a vacuum of assumptions. Of course your daughter isn't answerable to influences that took place whilst she was a 'minor' in the legal sense but, by the same token, and if phrased subtly, she might be open to shedding some light on their previous interactions in general terms.
- You also mention that DD's dad is 'fuming', but nothing else more on that front. Given that his own brother has created the issue, has he had any insightful input other than 'fuming'? You'll obviously be the judge of whether this line is worthwhile pursuing but if he has any sense of responsibility he needs to bear it - but again (and at a completely uneducated guess) it may not be worth pursuing.
By which process all this should be handled remains a big question. The stakes are high for everyone so the facts need to be established as best they can be under the circumstances.
In summary, you seem totally alone in bearing the responsibility for a situation that cannot remain as it is, no matter how uncomfortable it may be for all in the short term. For what it's worth (and I'm no expert) I would suggest that both you and your daughter ABSOLUTELY avoid engaging directly with brother-in-law on the matter until you're in a clearer position. Purely at face value, and ONLY if you're comfortable with it and think it might be of benefit in establishing a broader evidence base, perhaps you might dig a little deeper (sensitively no doubt) with DD to establish where this all stems from.
A lot of words but not much help I'd wager. Other than that, I couldn't possibly comment (I'm not joking either - others clearly have..)
Best Wishes