B737 First Officer reported their aircraft began sliding uncommanded towards another aircraft during taxi out. The Captain had to abruptly apply the brakes to stop the aircraft; causing two flight attendants to sustain injuries.

Narrative

After pushback we started the number 2 engine. The Captain elected to just start the single engine before we started our taxi out. We were planning [Runway] XXL so I believe we were both expecting to make a right turn out of parking. There was also ice buildup on the wings and active precipitation so we expected we would be going to a de-ice pad on the west side of the field. There was also snow/slush on the ramp. However our clearance was to make a right 270 to head to the east end of the ramp; to ultimately go to one of the north de-ice pads. We had traffic to the left of our aircraft which required the right 270.As the Captain started the taxi he noticed/felt that the aircraft was starting to slide toward the parked aircraft at the gate to our right. He consequently stepped a little abruptly on the brakes to bring the aircraft to a stop. After this action we received a call from the flight attendants that one had hit their head and the other hit their hip due to the stop. We then cancelled the taxi clearance with ramp and informed them we may need to return to the gate. We then called ops to ask if we could return to the gate and have a paramedic meet us there. Both were granted to us. After we pulled in; both flight attendants were seen by the paramedic and they determined together that they were good to continue.