I went to a very good school when I was in grades 7-10. In grade 7 we started algebra. First we started with basic set theory, then we covered the axioms of equality, ordering, and field axioms (we didn't call them that though). We also were forced to solve equations step by step providing every axiom we used in each step (which at the time I despised lol). It wasn't until I started studying Analysis and Abstract Algebra that I realized how "rigorous" our intro to algebra was.
We moved and my sister goes to a not very good middle school. She's in grade 7 now. I want her to have a very good foundation in math like I did. I'm prepared to teach her, but need a book that she can at least look over what we cover and do exercises from. I also need it as a guide, since I don't remember the order in which we covered the topics back when I was in grade 7.
Just to summarize:
I want a middle school level algebra book that covers some set theory and "axioms of algebra", requires some rigor, and hopefully includes some harder problems with the routine ones.