Poor Relatives receive 50k inheritance; plan to buy BMW

[captZEEbo]

Not actually relatives, but my serious girlfriend's sister/husband.

They are receiving 50k in $400/mo increments, and they plan to save up for a really nice BMW. However, as it is, they are renting an apt, have a 3 yr old, struggling to pay all the bills, in debt to their parents about $3500, and I think their annual salary is around 20k? (one is in school still).

Possible pertinent info: their family is all bad with money, and they view me as some financial wiz I think; the sister/husband are planning on seeking my advice as to where to invest the $400/mo until they can afford the BMW.

*Is there any tactful way to say that buying a BMW is the worst possible idea in the world?