Its alot easier to just get a little more cut and leaner looking.
Especially if you dont have much experience with working out or dieting, and you start on a good program.
Your quite light for you height, and I would expect you could put on 20-30 pounds this year if you eat and lift hard.
But shes definelty going to win in a time span of only 3 months.
Your going to have to eat like a motherfuka, and thats going to hurt.
6k+ calories a day of clean food if you want 15 pounds of quality muscle (and a few pounds of fat as well) in 3 months.
Thats about what I eat when I bulk, Im alot larger than you, and its a pain for me.
Milk will be your best friend, get gallons of whole milk, try to get atleast 1/2 a day down.
Your going to need to make it something more like 10 pounds in 3 months.
Also at her height she is pretty much the correct weight. I expect she has very little fat to lose at 108 and 5'1.
She needs to work on building some muscle as well as trimming what little fat she has.
This contest is complex, she needs to lose mabey 5-8 pounds of fat and gain about 5 pounds of muscle.
So going off her BF% would be a decent indicator.
But for you, youll need to check both the scale and the BF% to see how much muscle and fat youve gained.