A seven-member High Court jury, following a five-hour discussion, also convicted Ada Tsim of shooting two other relatives with the intention to cause them serious bodily harm.
The court earlier heard that Tsim, 47, had pulled out a gun and shot the four victims at Quarry Bay Park in June 2018.
It was also told that the former body guard had disagreed with her aunts and uncles over the way a property matter was handled.
While two of the victims – a 75-year-old man and a 63-year-old woman – survived their injuries, the other two, aged 80 and 62, died in the attack.
In wrapping up the 10-day trial on Wednesday, Judge Maggie Poon had instructed the jurors to consider whether Tsim had lost her self-control and had been unable to make reasonable judgements, due to her drug addiction and serious depression.