Eta:I don't think I answered your questions, yes a sippy cup is fine and I think cows milk would be ok, but not for all feeds at this age, but certainly one 'feed' could be any dairy such as yoghurt.

How many feeds in a 24 hr period is she having?At this age, I think it us fair to offer solids before breast and go from there. She might not need any feed replacements (ie milk or formula) if you can get her to eat more solids. Then if you cut back to only 2-3 feeds a day you might feel happier about breastfeeding for longer or at least long enough for her to go straight onto milk.
The milk/food ratio thing doesn't cease to be an issue when you're offering cows mlk though...even at 2 many people find their toddler drinks more milk than eats food...

I would just concentrate on the solids before feeds and work with that for now...unless you're completely over breastfeeding????