The most important thing is to make sure she doesn't get dehydrated.  If formula will stay down, that's good.  Otherwise, try watered-down juices or pedialyte.

Keep up with the fluids - water, and if (I asume not) you are breastfeeding, with that. As the formula is staying down, offer that sparingly. The foods to reintroduce with are BRAT:

• Bananas
• Rice (eg dry ricecake or baby-rice)
• Apple (eg puree)
• Toast (dry).

Steer clear of dairy for a few days, and of sugary and fatty foods.

If she shows interest in food, then tempt her with a little of something safe (from above). If she shows no interest, be led by that and just offer drinks.

Hopefully she will throw this bug off as fast as her big brother.