Is the failure to maintain ties of kinship and the severing thereof considered a grave sin, especially after swearing an oath not to contact them due to their failure to fulfill a trivial request?

It is better to expiate your oath and cease shunning your niece, due to the animosity and severance of kinship ties that result from it. As stated in the Hadith (definition): "If you take an oath, then see something else as better than it, then do that which is better and expiate your oath." Abu Dawud and Ahmad said: "Except for an oath to sever kinship ties," meaning there is no righteousness in it.