This is not typical behaviour for a 3 year old or in this case one that is on their way to being 4. The info you gave about the two older sisters tells a lot more. I would mention it to the mom with the idea she may not be aware of what might be happening at home or doesn't realize what is happening. Also it could be the child's way of getting attention in that if she bugs her older sisters mom or dad plays with her. If you can get the mom on board then there is a chance you can get behaviour changes. No parent wants to have their child left out nor do they want their child to be a bully but both things are happening here and it is the child that is suffering. Even if it ticks the parent off royally they need to know so there is time to make changes before the child would start school since that kind of behaviour would not be tolerated. She is beginning her pre-school year. Can you imagine what this child will be like come next Sept in JK! If nothing else you can work on the social skills at your house but start with her behaviour. Just as you would make a child that annoyed their friends or bit, hit, teased shadow with you for the day you may want to try that. All the time she is just standing with you while you make yourself busy (note I didn't say play with her) you can be talking about the play of the other children and ways that she could join in in a positive way and what kinds of play would make her friends not want to play with her. If no one has actually taught her these things then it is only partly her fault so to speak. At least if she is shadowing she isn't bugging anyone - except you.