Penalty for attacking ally?

In MTW, your king would lose 2 Influence for backstabbing an ally. Not that this would prevent the AI from sticking it to you twelve ways from Sunday, but it was nice to know there was at least *some* justice.

Is there anything similar in RTW/BI?

CountMRVHS

The short answer is, none noticable.

Long answer: The ally is more likely to backstab you, the alliance model is such a joke in RTW that it's nearly pointless. Backstab away.

Not nearly pointless, actually pointless.