This is a very tough one, because as a parent lying about a serious issue to a child for any length of time is difficult. They are small, smart, and they smell bullshit at those ages. They can tell something is wrong just by your body language.

But he did violate your sister's wishes. If it causes her pain to see him you should both honor that and stay away. He was wrong.

However, the death of a spouse is fucking devastating and creates all sorts of powerful emotions. Some of which is being misplaced onto your husband. From you and your sister.

If the children value your husbands presence and love, it will be very important to let them see him as the loss of two male role models simultaneously is going to leave some marks.

So a little NTA and a little NAH.