"The siblings are too fat."
Translation:La gefratoj estas tro dikaj.
It seems as a general rule that the more positive is the root. For dik-, remember that for most of history, being fat was a sign of privilege and rank. It's only recently that the availability of food has grown to the point where it has lost its positive feel.
However, don't take my word for it. See for yourself. Go to http://www.esperanto-panorama.net/vortaro/eoen.htm and browse through the mal- entries.
Here are a selection of "bad" words that start with mal-:
scold, cowardice, unseemly, vicious, nasty, awkward, bleak, stale, weakly, deserter, crude, lewd, slut, blameworthy, shameless, hindrance, slothful, constipated, awkward, repulsive, insubordinate, quarrel, submission, abasement, poor, fool, sick, famine, neglectful, antipathy, superficial, arrogant, infertile, decay...
Although, to be fair, there are some "good" words that start with mal-: frankly, generous, sober.