That adopted daughter was 20 years old at the time. Not 7. Sure, it is plenty weird and possibly quite sordid (especially given Allen's age at the time), but it isn't evidence that he also molested the 7-year old adopted daughter. Apparently, he never molested the older daughter. They went on to marry and they are still together.
To me, a man of 56 starting a relationship with his 20 year old adopted daughter (adopted 'stepdaughter' I guess, he wasn't there yet when she was adopted) is a pretty selfish and shitty thing to, indicating that the guy is somewhat oversexed (which wasn't exactly a secret to someone that knows his movies) but it isn't something illegal, and doesn't give us any indication about him being a pedophile or something like it. It does indicate that this guy shows selfish behavior that is outside of the socially acceptable norms. It makes the story more likely, but that is about it.
Why would the adopted girl lie? Well, that is a hard one. But it seems that a good portion of such accusations are not true. The ex was really mad at him (with very good reason), so who knows. The fact that they are 'appealing' to people to treat Allan as if he was guilty does indicate that they have gone off the track, at least somewhat. The case wasn't pursued, legally. Presumably because there wasn't enough evidence. Which means that there was no way to 'know'.
So, the 'facts are' that we don't know. Why would you be keen to judge? With Polanski, I could understand you judging (because in that case, it does appear that there is plenty of evidence) but in this case, you should not.
I get that, by conventional standards, his behavior was pretty 'immoral'. And I get that it being that, you feel you know for sure he is also a child molester. Just as you probably feel that if gay people are allowed to marry, nothing will stop people from marrying 1 year olds and sheep. But for me, the one thing isn't evidence of the other thing.
@ Elle: Yes, that is common. But whatever the reasons for the lack of proscecution, fact is that he wasn't convicted and that we don't know. We don't know why Mia Farrow agreed. Maybe she thought her adoptive daughter's story might not hold up. Or maybe it was just compassion with her daughter. And now, we can't know. Because there will be no evidence left outside of her testimony and, after all this time, and her being on the 'side' of her adoptive mother so clearly, that testimony is probably not enough. Not enough for them to convict, and not even enough for us to know. With OJ, we pretty much 'knew'. Amanda Knox? I'd say we have a pretty clear idea (well, she is convicted again, now). Polanski? Fairly clear. This one? It is just guessing.
My guess is that he did it, btw. But I don't know.