Moderator, please edit the original post to remove all mention of names.
As a minor, you are a little bit powerless to act, but your dad and/or your grandparents can.
The first step is to bring the mother to family dispute resolution with the view to instate a parenting plan which enables your father to see his children on a regular basis. Under the Family Law Act 1975, all kids have a right to know, be cared for by and spend time with both parents.
If the mother refuses, your dad will be issued a s60i certificate which will enable him to pursue the matter through court. If the cost of court is a deterrent, please let your father know that he can self-represent and that the resources and support available to self-represented litigants is vast - including this forum. With that said, only about 3% of all parenting matters end up in court, so it may never reach a judge anyway.
Your grandparents, as people concerned with the child's well-being, are also entitled to take these steps to spend time with their grandkids.
Try not to focus on the fact that the mother has five kids to four different fathers, all going through the same struggles as your dad. Courts aren't interested in the personal choices of parents, just in the children's right to see both their parents.
Feel free to ask more questions, but remember that it is critical you keep the details of your situation as anonymous as possible for your own benefit. It is a court requirement that a certain level of confidentiality be maintained at all times because the matter involves minors. Refer to your brother and sister as 'the children', don't provide location details, etc. These provisions are outlined in the Family Law forum sticky post. I hope this helps.
