<human>: Condense the text into a few key points

My domestic partner/SO of 6 years has a brother Allan (33 years old). In October their father Timothy received a facebook message from a woman claiming to be the mother of a child sired by Allan. When Allan was in jail in September 2012 he was forced to take a paternity test which confirmed that he is indeed the biological father of this 4 year old.

Allan became furious with his father for speaking with the woman, saying he refuses to pay child support even though he acknowledges that he is the biological father, and forbidding Timothy from speaking with the mother or the child.

In February, Timothy came to stay with us, and told Allan that he has now disclosed the existence of this child to my SO. Allan threw a fit, and said he will "never forgive" Timothy for telling my SO about the kid.

Now my SO's and Allan's mom Roberta does not know about this kid. Roberta is stating that she would like Allan and herself to come and visit us in April. 

I have insisted to my SO that Roberta cannot come visit us with Allan unless Allan agrees to tell her about the child, and if he refuses to tell her and decides not to come, then my SO will tell her when she is here.

I think it is unfair for me to be asked to lie to Roberta about the existence of her grandchild at the request of her tantrum-throwing son Allan while she stays with us. My SO agrees, but knows that Allan will probably throw a giant fit.

Is it fair of me to set this boundary? What should we do?
<bot>: My SO's brother has a child he won't tell his mom about. Should I let brother and mother come stay with us while knowing about the child, or should I insist that the secret come out?