Half siblings!

So my baby daddy have 3 other children to two different mammas but all 3 of those children live over in the UK and both him and I plus our daughter live in NZ. His eldest children are 13 and 17. My daughter has nothing to do with him by his choice and from what I am aware of he has hardly anything to do with his daughters over in the UK and at this current stage they do not know about my daughter as he refused to tell any of his family but I have found his daughter and her mother on Facebook and am wondering if I should send either of them a message letting them know about my daughter so that if the girls chose to they can have some kind of awareness and contact with their half sister. I don’t know much about her father as it wasn’t anything serious when I got pregnant and have no contact with him at all. What would you guys do?