A Santa Rosa woman injured in a personal watercraft crash on Lake Sonoma Sunday collided with her younger brother on another Jet Ski-style craft.

The 23-year-old was at the lake with family members, including her 17-year-old brother who apparently was visiting from New Jersey, said Sonoma County sheriff's Sgt. Ed Hoener.

The siblings were riding near the boat ramp by the Warm Springs Dam when they hit each other, Hoener said.

It remained unclear Monday what led to the collision.

"We don't believe they were full throttle, but they were fast enough to damage the boats and cause a sizable head injury for her," said the sergeant.

They were knocked into the water and the woman apparently was unconscious, at least for awhile.

Both were wearing life vest — required by law — and the brother held his sister's head out of the water.

"He was there for her," said Hoener.

A passing boater stopped to help and got the woman to the dock.

Sheriff's deputies who patrol the lake on busy weekends responded to the 4:25 p.m. call for help as did Geyserville firefighters and a Bell's ambulance crew.

She was flown by CALSTAR helicopter to Santa Rosa Memorial Hospital, treated for the head injury and released, Hoener said.