I am sorry you are going through this Redpepper and Mono. I know it sucks to be so heavily judged by family.
Give it some time and let cooler heads prevail. Maybe it can work into grudging acceptance.
A knew a white girl who got pregnant by her black boyfriend (in southern USA). Her parents were angry and mad. Fast forward two years later and the parents love this guy and say they feel proud to have hom in the family. Sometimes they have to learn their prejudice was not right.
I tend to approach issues like this very analytically. They have to learn that this is not cheating. And they should recognize that Mono is taking nothing away. Would they behave as such if an uncle was staying at your house? Hopefully they can eventually appreciate that there is another adult to help watch over the child.
I wish you well in dealing with this.