I don't see anything in your account that screams bipolar. It's certainly possible, and some of the things your ex did and said seem "off," but that does not necessarily mean bipolar. He could have a different mental illness. He could not have one at all. It would take a lot of time with him being face-to-face with a therapist or psychiatrist to figure that out.

Given that, I wonder what the benefit of you contacting his sister or father would be. I don't think it would be helpful for you to speculate to them that he is bipolar. I suppose you could explain some of his erratic behaviors and just say that you are worried. But if he is not estranged from them, I have to think that maybe they have experienced some of this behavior first hand. It is unlikely that they are totally in the dark.

It sounds like you already told him you think he may be ill and needs help. I think this was really all you could do without putting yourself back into his life. And if you are going to continue a court claim against him, it may be be unadvisable to keep contacting him.