Infidelity is not in the same category as the others you mentioned. Sure cheating sucks, but it's usually a sign that something was wrong in the first place. So while I may not be thrilled with the choice, I'm not going to hold it against someone.
You're way more open-minded than me. Infidelity is bad enough with no children involved. Once you involve kids, you teach them:
1. to grow up instantly, childhood is a thing of the past.
2. the people whom he/she loves most of all cannot be trusted at all
3. the new partner may well take priority over the child
4. once he/she enters a relationship on their own, they will likely be very distrustful, because trust is easily broken.
Seriously, I don't understand this philosophy of "my sibling will always be welcome at my home" Sometimes your sibling acts like an a__ and needs to skip the family holiday gatherings if he/she is going to force a new relationship down everyone's throat. By forcing it, he/she is telling the rest of the family to subvert their own feelings.

