Since it is undergoing peer review again, in general, you should be able to include the new papers. If you do this, though, highlight both the new papers and the changes to your conclusions in your author response / revision note, so that the reviewers know to pay particular attention to them and give them proper review - if you don't, they might not notice, and skip over important things because they think they already reviewed it.
It's common for reviewers to ask for new analyses or even experiments in revision, so this seems like a relatively normal kind of update in a revision cycle.
If in doubt, e-mail the managing editor. One way to do that productively is to make a specific proposal - e.g. include the new papers and highlight in the author response letter - and ask if that is an appropriate way to handle the situation for their journal.
A note that doesn't seem applicable to your specific situation, but may help others finding this answer: the exception to this is when you get what many journals call a "Minor Revision" decision, where only the editor will see the revisions - it won't go back out to reviewers. In such a situation, I would contact the managing editor and notify them of the situation. They may be able to handle the revision as a "Major Revision" and get reviews for it.