First, Bostrom is very explicit that the conclusion of his argument is not "We are probably living in a simulation". The conclusion of his argument is that at least one of the following three claims is very likely to be true -- (1) humans won't reach the post-human stage of technological development, (2) post-human civilizations will not run a significant number of simulations of their ancestral history, or (3) we are living in a simulation.

Second, Bostrom has addressed the objection you raise here (in his Simulation Argument FAQ, among other places). He essentially flips your disjunctive reasoning around. He argues that we are either in a simulation or we are not. if we are in simulation, then claim 3 is obviously true, by hypothesis. If we are not in a simulation, then our ordinary empirical evidence is a veridical guide to the universe (our universe, not some other universe). This means the evidence and assumptions used as the basis for the simulation argument are sound in our universe. It follows that since claim 3 is false by hypothesis, either claim 1 or claim 2 is very likely to be true. It's worth noting that these two are claims about our universe, not about some parent universe.

In other words, your objection is based on the argument that if we are in a simulation, there is no good reason to trust the assumptions of the simulation argument (such as assumptions about how our simulators will behave). Bostrom's reply is that if we are in a simulation, then his conclusion is true anyway, even if the specific reasoning he uses doesn't apply. If we are not in a simulation, then the reasoning he uses does apply, so his conclusion is still true.

There does seem to be some sort of sleight-of-mind going on here, if you want my opinion. I generally feel that way about most non-trivial uses of anthropic reasoning. But the exact source of the sleight is not easy for me to detect. At the very least, Bostrom has a prima facie response to your objection, so you need to say something about why his response is flawed. Making your objection and Bostrom's response mathematically precise would be a good way to track down the flaw (if any).