My sister and niece are spending a night in Frankfurt later this month on their way to London.
They will have about 24 hours....what can they do
sightseeing wise in that time without getting lost?

A lot, what do they want to see and do?

Neither has ever been to Germany and will most likely never go again....flights were done with miles and this was only way it worked....so most anything.....old stuff...don't care about financial center etc....just older historical stuff....sister said something about a day boat ride but I don't think the timing will work for that....so any and all suggestions are appreciated...we just don't want them to get lost!!!

Train runs right into airport and from there you can go anywhere! Zoo is supposedly pretty amazing.

There is really not much in Frankfort to see. I would suggest a short train ride to Heidelberg, its about an hour each way. You can book tickets on the site below. Only one train change at Frankfurt main train station. Enter Frankfurt Airport and Heidelberg as your stations.

https://www.bahn.com/i/view/USA/en/index.shtml

Keep in mind that more and more young displaced Islamic males are floating around Germany. I know quite a few folks who have changed their travel plans through Europe as a result. Not being hyperactive about it, but everyone should just be aware - especially unaccompanied women.

If there is time, take the train to Koln (Cologne) and se the cathedral.