I took care of her. I've always been there for her, godmother to her daughter, helped her move from rental properties previously, went in holidays together. I just think it's really sneaky if I'm being honest. I can understand not telling everyone but I'm her sister.
If your attitude is that she 'owes' you total disclosure on all her activities because you've 'always been there for her', then I think you have the answer to your bafflement. She wants a bit of space from your input.
In the nicest possible way, also isn't there much going on in your own life that a sibling's house purchase looms as some major event?
I'm fond of my parents and siblings, and talk regularly but I didn't tell them I was getting married, only told them I was pregnant at 20 weeks (and didn't tell them I was in labour until DS had been born by CS), didn't tell them we were moving countries, didn't tell them we had almost bought a house but pulled out when a big problem emerged late in the process etc.
My parents are terrible worriers who are capable of imagining the worst in virtually any scenario, so it's kinder all round to present them with a fait accompli, and also I get irritated by the handwringing, as if normal life events are some kind of opportunity for doom.