The typical rule (e.g. here) is that number agreement for a sentence with an interrupting phrase should be with the item before the interrupting phrase. For example:
- Joseph needs to buy some new clothes
- Joseph, not to mention his brother Reuben, needs to buy some new clothes.
That is, "needs" remains in agreement with the singular, and does not change to the plural "need". And even if add a word as follows:
- Joseph, not to mention his brother Reuben, needs to buy himself some new clothes.
Still, I believe the rule is to focus on that singular "Joseph", so the word added is "himself" and not something like "themselves" (I'm not sure even that's correct, but that's not important here. The core question is, whatever the added word, does it remain singular?)
But suppose I really push it, as follows;
- Joseph--not to mention his brothers Reuben, Simeon, Levi, Naphtali, Issachar, Asher, Dan, Zebulun, Gad, Benjamin, and Judah--needs to buy himself some new clothes.
(I've switched to an em-dash for the parenthesis only because the list commas would confuse)
Now the interrupting phrase dominates, and there's a temptation to want to move to:
- Joseph--not to mention his brothers Reuben, Simeon, Levi, Naphtali, Issachar, Asher, Dan, Zebulun, Gad, Benjamin, and Judah--need to buy themselves some new clothes.
While that still breaks the rule, there's now a nagging feeling that in such a case the rule is wrong!
Now in practice, I would deal with this by rephrasing. I'd do that by moving the main mention of the brothers outside the interrupting phrase, leaving the phrase as a mere elucidation as to said brothers names. Something like:
- Joseph and his brothers--Reuben, Simeon, Levi, Naphtali, Issachar, Asher, Dan, Zebulun, Gad, Benjamin, and Judah--need to buy themselves some new clothes.
In one sense that's exactly the right thing to do, because the very fact that I see the big interrupting phrase as so dominant suggests that in terms of what I'm trying to say it should not be an interruption! But in another sense, I'm taking the coward's approach by avoiding the question entirely. :-)
So, is 5. actually wrong and to be avoided?