Hi Bill,
Could you post a digital image of the letter and the funeral notice somewhere? (the list doesn't allow attachments). Please forgive me, if you have done so, I have only now started to follow this thread. With just the information I have read so far my assumption would be that one or several members from your family "emigrated" from Lower Saxony / Hannover to Dortmund in the 19th century (quite common and more likely than the other way round) but stayed in touch also after 1900.
It would then be normal that letters were sent from one place to the other. As for the funeral notice, please note that while "city, state" is a normal way to describe cities in the US it is not very common here. The format would rather point ot alist of two cities. So, probably the funeral notice represented both parts of the family, the one in Dortmund and the one in (the city of) Hannover; and the funeral notice just indicates that relatives lived in those two cities, a quite common practice for these type of notices.
As mentioned above, if you could post digital images somewhere we could have a look.
Btw. is the family name "Mädel" confimed? You may know that "Mädel" is a variant of "Mädchen" or girl, so would not necessarily be the name.
Regards,
Nils