Is it obligatory to maintain kinship ties with relatives who disowned the questioner and insulted her mother, and with her brother who assaulted her?

The default is the obligation to maintain ties of kinship, especially with unmarriageable relatives (dhawu ar-rahim al-mahram). This tie is not severed by their mistreatment; for the one who maintains ties is he who connects with his severed kin. A Muslim will continue to have support from Allah against those who mistreat him as long as he maintains ties with them. However, if maintaining the tie results in certain harm, then whatever wards off that harm is waived from the tie. In such cases, merely making phone calls or using other various means of connection may suffice.