Revised Answer:
Is this guy flat out evil or something? I mean, is he some sort of perverted murderer, child molester, drug dealer, illegal guns dealer...Is this guy a hardened criminal???
Honestly, what in heavens name is wrong with a guy wanting his children for his birthday??? It really just does NOT matter if he asked for them before, he's asking now. This is the children's FATHER for goodness sake.
Just because you CAN make things difficult and nonsupportive of the child/father relationship and just becuase a Mom wants to have complete control of her children as if it's a source of power and the children are her sole "property", does NOT mean she is justified.
Unless this guy intentionally put the children in harms way, unless he's proven to literally neglect the children in his care, there is absolutely NO reason he should not be allowed to share his birthday with his children. I'm sure your sister does. And as I orginally posted, the guideline is the MINIMUM visitation, it was put in place for the petty, ridiculous, immature folks who choose to continue their irrational disputes with each other onto their children. (Though I realize it's really easy to be that person, I, myself, have this little song and dance with my exhusband who desires as little time as possible with our children and this "minimum" thing is MORE than he takes advantage of.)
So, my vote is, unless she has a legitimate fear that her children are going to be harmed, neglected, or hidden away, OR the children are afraid of him because of his own actions, she has no real reason to keep them away from their father. In my estimation, the more she can do to foster a good relationship with them, the better a mommy she is.
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I've given you links to read how the Utah Courts have it written. This is by no means ALL the visitation possible. This simply means that if the two parties cannot agree, this is the minimum the custodial party MUST ALLOW the noncustodial...which also doesn't mean this is the full amount the noncustodial will put to use. You'll see here that birthdays and holidays are alternated each year.
OVER:
http://le.utah.gov/~code/TITLE30/htm/30_03_003500.htm
UNDER 5:
http://le.utah.gov/~code/TITLE30/htm/30_03_003505.htm