Is it obligatory to maintain ties of kinship with brothers who frequently harm our father and threaten to kill him? And what are the means of maintaining ties if visiting is difficult? And is giving charity on their behalf sufficient to avoid their harm?

It is permissible to shun and sever ties with these brothers if maintaining relations with them leads to harm and injury. However, if you limit your interaction to the bare minimum, such as greeting them or speaking to them on the phone in a way that does not lead to evil or harm, then that is good.

As for giving charity to them, it is permissible, especially if it is hoped that it will lead to their reform and guidance. Likewise, giving charity on their behalf is also permissible.