Thank you all for your excellent suggestions.
@Lenny_Ogg unfortunately the mod seems to be outdated. It broke my vampire powers/weaknesses. No matter how many weaknesses selecten (0 or up to 8) I could not hit ok after selecting them.
@EnkiSchmidt Thanks for pointing out this usefull feature in MCCC. Unfortunately vampire powers and weaknesses don't shop up in this list. I will use this feature in the future if I ever need to change traits, but for this issue it won't work.
@LuvtehGeeks I was not able to figure out how to add specefic weaknesses (or powers) with mccc. Only to reset them seperately.
I have found a workaround. I rolled back to a previous save about 5 sim days ago where my sim was rank 3. I added all the powers of the lower ranks and the 2 weaknesses I wanted. I let her rank up to rank 4 again. Then I used the cheats to add the sun resistance power. So right now my sim has 2 weaknesses and sun resistance. I can't add any more powers with the ingame UI because then I would be forced to take up more weaknesses but that's fine. Let's see it as a trade off for getting the sun resistance in combination with 2 weaknesses.
The only downside when rolling back was that her infant daughter no longer had the crawl milestone due to rolling back to a previous save but the little girl managed to earn this milestone a second time really fast. :)