Going to Munich, what should I not miss seeing?

I still have fond memories of this when my wife and I visted Munich

https://en.wikipedia.org/wiki/BMW_Museum

That trip we visited museums in London and Paris and the BMW museum still stuck in my head. It may have changed since then, that was quite a few years ago.

Well rated at tripadvisor as well

http://www.tripadvisor.com/Attraction_Review-g187309-d190264-Reviews-BMW_Museum-Munich_Bavaria.html

Not sure when you are going but if you are going to Munich at the beginning of May, then the High End is the THE place to go in Munich:
http://www.highendsociety.de/english/highend/highend.php

Munich is also full of record stores:
http://muenchen.cylex.de/branchenbuch-muenchen/L1c1-D_O1c1-M%DCNCHEN-P1c1--N1c1-SCHALLPLATTEN.htm

Also, this is the official site for Munich tourism:
http://www.muenchen.de/int/en/

Rick Steves European travel shows on KCET are among the best. He's been everywhere. Here is his site with advice:http://www.ricksteves.com/

All the best,
Nonoise

Take in the Hofbrauhaus for sure.

I fondly remember my trip to Munich in 2010. Like Albert Porter wrote, you must go to the BMW Museum. It really is fantastic. About a 1/2 day's worth excursion.
The 2nd place I really loved was the Deutches Museum - this is a museum of natural history. Fantastic! Beautiful location as it alongside a river. The museum has a tall tower with massive 10-story tall hygroscope to measure humidity! You can climb all the way to the top & get a great view of the city.
What I also did was travel up & down the subway to all my locations & it was very educational (I can read/write/speak German so it was not too difficult for me but even for non-German speaking folk just using some common-sense like knowing which direction you are headed & holding a subway map you can find your way around very easily).
Of course, you must see the Marienkirche at Marien Platz. In the summer that whole platz (square) will be a magnificient spot - outdoor dining, people milling around, street performers & beautiful weather. Aaaaaah! your trip is making me jealous.....;-)
If you have some time to go a bit outside Munich you can visit the dreaded concentration camp Auschwitz. I did not have time but I would have loved to have visited it.
And, I'm sure that your relatives will guide you well....
Have fun! It's a beautiful city...