The fact that something is true doesn't automatically make it believable. Firstly, truth really is sometimes stranger than fiction. Weird coincidences happen. People behave dramatically out of character. Chance strikes families for good or bad. Life is random.
Secondly, if it really happened - and to someone you know, or even yourself - then the chances are whatever you write will be coloured by your knowledge of the ins and outs of the character details, the location the event takes place in, the effects and repercussions that are relevant to the story today. You will probably write something that is utterly clear to you, but lacking in the information that will make it live for a complete stranger.
We need concrete detail. It's not enough to say Uncle Bob's house, because that means nothing. We need inter-war semi, chocolate box cottage, country mansion. We need to know if it's crumbling or in perfect order. What is it made of - brick? stone? What are the windows like? Do they let in much light, or are they small and dirty? We need all the information that we might use if you were making Uncle Bob's house up.
We also need meaning. Yesterday I was talking with my mother about Call the Midwife. I knew that my mother had trained as a midwife in Edinburgh after the war and wondered what her take on the programme was. It turned out she hadn't bothered to watch, so that was a bit of research lost, but she told me a story about going to a home delivery and, on her way back to the hospital, leaving the placenta on the bus, and having to collect it from Lost Property. Great story - to me, because it's my mother, because I've heard it before, because it's family history.
It's a lousy story, actually, because nothing happens. She just collected the placenta from a mildly startled Lost Property man and took it back to the hospital. There's lots of meaning for me, but none for you. If you were to fictionalise it, you'd have to find a meaning somewhere - perhaps she learns to be more confident, perhaps she learns the dire consequences of being forgetful, perhaps she falls in love with the Lost Property man, perhaps she's mistaken for a mad axe murderer, perhaps she's blackmailed by Satanists looking for suitable material.
Which leads to the next problem - inhibitions. You may be reluctant to embroider a story that features a real family incident starring a real member of your family. You may not want to tread on toes. You may feel that it's cheating, somehow, to alter the facts. And above all, you may not realise that what's a great story to you (because it really happened to you or someone you know) might not be a great story to me (because I have no connection with it).
So tread carefully round the truth. It's almost certainly unbelievable, but not necessarily in a good way.