You’ll need to use mod_rewrite (RewriteRule and RewriteCond) directive(s) instead, as opposed to a mod_alias Redirect directive, in order to check the requested Host header.
For example, try the following near the top of the .htaccess file, before the # BEGIN WordPress section (the order of directives is important):
RewriteCond %{HTTP_HOST} ^foo\.org [NC]
RewriteRule ^events$ https://bar.org/events/ [R=301,L]
This redirects only http(s)://foo.org/events to https://bar.org/events/ and ignores any other requested hosts, such as subsite1.foo.org. Note that the trailing slash is omitted on the source URL (as in your later examples), but included on the target URL. Your first example omits the trailing slash on both the source and the target URLs?
UPDATE: The redirect should work whether or not there is a trailing slash
In that case, you should change the RewriteRule pattern to read ^events/?$ in order to match both /events and /events/ and redirect to bar.org/events/ in both cases. In other words:
RewriteRule ^events/?$ https://bar.org/events/ [R=301,L]
(Including the preceding RewriteCond directive as before.)
You will need to clear your browser cache before testing, since any erroneous 301 (permanent) redirects will have been cached by the browser. Test first with 302 (temporary) redirects to avoid potential caching issues.
Note that the Redirect directive is prefix-matching, so your previous Redirect 301 /events https://bar.org/events/ directive would also redirect /events/foo/bar/baz to https://bar.org/events//foo/bar/baz (note the erroneous double slash). To replicate the same behaviour with the above RewriteRule, but less the double slash issue, you could change the RewriteRule directive to read:
RewriteRule ^events(?:/(.*))?$ https://bar.org/events/$1 [R=301,L]
This would redirect /events to bar.org/events/, /events/ to bar.org/events/ and /events/<something> to bar.org/events/<something>.