Start giving her small gifts of appreciation. If she and/or your grandfather is/are Christian, get her or make her a small cross. Draw a picture of them holding hands by a church. Real religious stuff that works with their faith, or at least your grandfather's. If neither are faithful, get as close as you can to whatever you think he might have even a little faith in. If they say anything, just say you've been thinking a lot about life and that you are looking into things. If she gets nicer, she isn't evil. You will know what I mean. If she gets meaner, she is evil.
The religion has nothing to do with this, your kindness does. Evil doesn't understand or react well with kindness. Showing kindness that embraces the faith of another is a very powerful thing in families. Also, if she turns out to be evil, it will help with what I would recommend you do next. If she is evil, message me with the steps you took and how she reacted. I'll then tell you how to make a simple ward out of a religious item so they don't think you are using magick, but still being friendly.
If this doesn't work for you, then start learning sign language. Start using it for simple things when you talk, like asking for the time and/or eating or drinking. Then, when you tell your family that you love them, start using the sign for "I love you." This is a simple ward against evil. I've used it to ward off evil in the past, including evil people. One evil woman I once knew would fly into a rage when people used it around her. It won't get rid of her, but you will know she is evil.
Just be sure to use the steps I'm recommending. If they think or even suspect that you are trying to prove she is evil then they will not be as effective as you will be accused of being the one bringing in negativity. I hate to ask you to be deceptive, but sometimes you have to be a detective to get the evidence you need.