Is one who severs part of his ties of kinship considered as sinful as one who severs all his ties of kinship? And does one who severs his relationship with a person or a group fall under the meaning of "severer of the ties of kinship" mentioned in the Qur’an and Sunnah?

The severing of ties of kinship has degrees, just as maintaining them has degrees. So, maintaining kinship ties has degrees, some higher than others, and the lowest of these is abandoning estrangement and maintaining connection through speech. One who severs ties of kinship is not only someone who cuts off all relatives, but also one who cuts off some of those with whom connection is obligatory; such a person is considered a severer of kinship ties. For example, whoever cuts off one of his brothers is a severer of kinship ties, even if his sin is less than someone who cuts off all of his brothers.