Sharing is one of those things that needs practice. You may want to start by you sharing things with your granddaughter. Like a cookie. "I would like to share this cookie with you." While your GD enjoys the cookie, you could say "Gosh it feels good to share with you" Then ask her "will you share your cookie with me?" Then compliment her. "I like when you share." Use the please and thank you words with each transaction.
Have her parents share with each other. Have them share with you. Verbalize the transactions each time. Even compliment each other, even though you are all adults, respond with thank you. It may feel strange, but your GD will see and hear the way to share.
When she "takes" from the twins, ask her to make sure She asks for the toy first. Make sure she has a toy to share back with them. Of course they are so young, they may not understand right now, but this way your granddaughter is learning to use her words with them. And the twins will grow up in a sharing home.
Also we taught our daughter and the neighborhood kids to share for 2 minutes. Meaning if someone is really playing with a toy and someone else decides they want to play with that toy. The child would ask, Can I please share your toy in 2 minutes?" They either checked back in a little while or forgot about the toy. Many times, the other child would say, "you can play with the toy now"..
When she takes, gently remind her "that is not sharing". "show me how you share". Remember even 3 year old's use time out chairs, so she may need to be put in time out for not following these rules. Hang in there, you are a peach to care for your grandchildren. They are all very fortunate.