I believe fellow TAs are giving students answers and inflating their tutorial average, what should I do?

Is it possible that the other TAs, by having done the course before, have a better overview of the kind of questions asked in the exam (based on those of previous years), and therefore are better able to train students for those exams? That would explain the facts as well, without anybody consciously in the wrong (and it is always worth to explore such possibilities).

I can also understand that your professor is not actively doing anything about it as long as the difference is not too big. Some TAs are better than others, some groups are better than others, there is always noise and some differences between groups are to be expected. I was a TA for several math courses where we would grade weekly assignments of our own groups (that counted for 10% of the final grade, next to a 90% final exam). I would grade harsher than other TAs and my group's average of the homework would be lower than that of other groups. However, my group tended to do better at the final exam, I believe in part because they were pushed harder to make an effort. What I mean to say is that worse intermediate test results don't say everything.

Unless you have hard evidence that your fellow TAs are 'gaming the system' I would be hesitant to attach too much meaning to the present facts. If, besides this, their manners are unpleasant or the group atmosphere isn't right, the easiest solution would definitely be to avoid them by not TAing the same course next year.

Well, you could roll with the system and make sure the review does cover necessary material...

Also, can you change the course you TA for ? If not immediately, then for next time?

You say there are four professors teaching this class, have you tried reaching out to the other three?

At the end of the day - if the professor is aware of it and condoning it, then there's not much in the way of academic misconduct, but arguably a bad teaching methodology. If the tutors are acting against the policy set out by the professor and they don't care/aren't acting themselves, then there's little you can do about it I think. At the end of the day, you have done your due diligence and reported the matter to the professor.

If nothing happens and your tutorial groups suffer a significant grade disadvantage due to this, and the professor is still ignoring it, then perhaps a slightly underhanded way of handling it is casually mentioning this to your own tutorial groups. The students would undoubtedly be furious to hear that their poorer performance is due to other tutorial groups cheating!

A 'meta' remark. A lot of times issues like these are extremely difficult to prove and press forward, even if your fellow instructors are in the wrong. They often have implications outside the class, and often the party raising the issue (you) will simply be remembered as a 'troublemaker' (good luck trying to run a research project with graduate students/professors that you accused of academic dishonesty, let alone if they suffer disciplinary action). So there is the question of how much is this bothering you and how much you'd like to push it forward.