Raped as punishment? What gave you that idea? Punishment for what, for the death of the king of the Iceni? Boudica hadn't revolted yet, there was nothing to punish the royal household for.

In any case, Tacitus, our only source for the treatment of the Iceni prior to the revolt, directly contradicts the idea that this was either punishment or sanctioned:

quod contra vertit, adeo ut regnum per centuriones, domus per servos velut capta vastarentur. iam primum uxor eius Boudicca verberibus adfecta et filiae stupro violatae sunt

But it turned out to be the opposite [Prasutagus had left part of his kingdom to Nero at his death, in the hopes that his kingdom and household would be well-treated], such that his kingdom was ravaged by centurions and his household by slaves, as if they were captives. Now first his wife Boudicca was beaten by the lash and his daughters violated with defilement [stuprum means something like "disgrace," or "dishonor," particularly with sexual or lustful connotations--here it's a euphemism for rape]

What this means is that after Prasutagus' death his kingdom became a province, rather than a client state jointly ruled by his two daughters and Nero. His people's lands were plundered by military forces, sure, but the acts of violence and depravity that his household was subject to were the work of slaves, not free men. Tacitus blames the procurator of the province for confiscating the property of important men within the new province, but condemns the veterans settled at the colony at Camulodunum, whom he said thought that the whole kingdom had been given to them as a prize and treated the entire population as though they were slaves. This was quite clearly a series of acts of complete anarchy, not something officially put in place by any legal authority--while certain aspects of it were legally sanctioned (like the confiscation of property), all that other stuff was not. This Tacitus makes quite clear, and it's something of a big point for him--Tacitus tends to present Britain as being among the most exploited provinces, until Agricola fixed things. But the idea of rape as an official punishment used legally by Roman authorities is not only totally unattested but rather ridiculous--a Roman would've been disgusted by the thought of such a punishment, and they say so rather frequently