From a comment:
So if I'm understanding this correctly, sexual assault could be a very specific type of assault (e.g. rape), or it could be something altogether different than any definition of assault (e.g. voyeurism). In either case, sexual assault is defined independently of assault by the statutes of the jurisdiction in which the act takes place.
It really depends on the jurisdiction. It would certainly possible for a code to define assault and then say that if some certain sexual element is also present that it is then sexual assault. However, the codes I checked before submitting this answer tend to define sex crimes separately from assaults.
For example, in New York, rape is not in the "assault and related offenses" article but in the separate "sex offenses" article. There is no crime of "sexual assault" but there is "predatory sexual assault" (as well as "predatory sexual assault against a child").
New Jersey, on the other hand, has no crime called "rape." Instead, it has a crime called "sexual assault," the definition of which describes a crime that one might expect to be called rape.
Since definitions are so inconsistent across jurisdictions, it's nearly impossible to evaluate whether your proposed hierarchy is accurate without reference to a specific jurisdiction, or perhaps to a custom set of definitions, although that would invite accusations of begging the question.
Another thing to consider is that crimes are often defined in a mutually exclusive way. For example, one of several conditions for aggravated sexual assault in New Jersey is "the actor uses physical force or coercion and severe personal injury is sustained by the victim," while the corresponding condition for sexual assault is "The actor uses physical force or coercion, but the victim does not sustain severe personal injury." Clearly, the same act cannot satisfy both conditions.
I suspect that you're looking to the legal definitions to provide precision, but unfortunately for the US context, that's too much precision, as we've seen. This leaves us with the dictionary definitions of these terms, which are not necessarily precise and which are fluid. For example, people can broaden definitions for rhetorical purposes, or use words metaphorically.
Still, in that context, it is certainly reasonable to assert that rape is a form of sexual assault, and that sexual assault is a form of assault. But, also in that context, it is not reasonable to make a formal, rigorous, or absolute statement about the truth of those assertions.