How did you learn this and what is being done about it?
A 12 yr old that is molesting other younger children is in serious need of help and this cannot be ignored.
The four and five year olds also need counseling to help them understand that this behavior is NOT acceptable from anyone.
If nothing is being done about this situation, call your Child Protective Services immediately before any more damage is done to these innocent children.
Professionals have ways of finding out from such young people exactly what has happened without making them feel that they were wrong or at fault.
It has to be rough reporting a family member, but you must think about your grandbabies being raised where the actions of their cousin is swept under the rug, if that is the case.