That's an interesting piece of the past, albeit a bit hard to read.
It is definitely addressed to Johanna Thavonat and calls her by the nickname "Hansi".
From what I could decipher, he (the one writing the letter) is very happy for her about a certain decision, and I am pretty sure he is talking about her marriage, as he talks about her new "Hausstand" (household) and on the last page he tells her to greet her "Gatte in spe", meaning the "husband about to be".
Then he talks about his wife being sick, apparently of a severe depression of some sort if I understood it correctly ("Nervenverstimmung und Melancholie" = nervous affection and melancholy).
You may also be interested to know that the letter is signed "Großpapa" (grandpa), but I can't read the name after that. So this could have been her grandfather writing and congratulating her - but he uses the polite form (Sie instead of du) and also talks about his daughter, which should be her sister? I am a bit puzzled by that...