My family (which consisted of my parents and 2 siblings) used to live in a relatively small town in the east of Queensland. It was a nice area close to the corner store and we had plenty of neighbours. Most of the houses that surrounded the town were old queen slander houses, so they were mostly elevated or a bit worn down.
My family and I were only renting the house while we searched for permanent living conditions, but within the time we stayed at the house everything felt "off". I remember my Mum waking up occasionally and talking to herself, saying stuff like, "Who's there please?" At a young age, hearing my mum say that made me terrified.
Another incident that regularly occurred was the lights flickering. Mum and Dad always said it was just the old wiring and not to worry about it too much, but those words could never calm me.
One significant event that occurred while staying in the house was when we were putting a new door up in the garage. My older sister was about 12 at the time. She ran out of the house and came over towards dad. He was currently standing on a ladder screwing the door onto its hinges when a gust of wind blew the door down and on top of my older sister. I don't remember much of what came afterwards but it was pretty hectic. I remember my sister's head was bleeding and mum calling an ambulance, but that's it. However, it wasn't a windy day. With everything that had happened previously, I just couldn't bring myself to believe that it was he wind. My sister now has a scar on her forehead (a bit like Harry potter's but straight). We moved from the house and now live not far away, but in a nicer area.
About a year ago though, my older sister (who is now 17) was experiencing some more weird encounters. The first thing that ever happened was small stuff. She had an Australia Day flag in her room and it would move as if there was wind even though the fan was off. She even said that she once saw a white figure in the corner of her room one night. I never really believed her until one night when I came home from dance. In the dance we used helium balloons so I was given one to take home and practice with. I left it in the corner of my room before going to bed. My sister woke up in the morning crying because she said she couldn't breathe. She had the balloon string wrapped around her neck. I was petrified and never brought home a balloon again.
Not long after mum went to go see a clairvoyant. The woman confirmed that a spirit had followed us from the old house that we lived in and that the only way to get rid of it was to place salt in corners of my sister's room and then dispose of it far away. We did as she said and have never had any experiences since.
The door blowing over... May have been a very random breeze... Or maybe it was barely balanced and the vibrations caused by your sister closing the door caused it to fall... As far as the balloon goes... Did you and your sister share a room? IF so then maybe during the night the balloon drifted to her side of the room and the string got wrapped around her neck as she rolled over.
I am not saying I do not believe you, but EVERY possible natural cause MUST be ruled out before the paranormal may be considered.
Respectfully,
Rook