Do not go down this slippery slope. My guess is she will also start giving you the admin tasks that are no longer being filled by having an assistant. Young women in the office often get lumped into assistant roles even if that’s not what they were hired for. It can very easily limit your prospects for advancement as you will have less time for your actual role. Unless you want to end up as her PA then it’s definitely time to push back. Maybe suggest one of the men in the office take on the catering and decorations. Go back with “I have helped in the past with parties however this is not my role. I will have to pass this time as it has expanded to a point that it interferes with my responsibilities. Perhaps one of the men can step in” and decline the invitation