Feelings of Guilt
Knowing that Lisa is Kumala's biological child actually makes Peter's view towards Lisa change drastically. At first, he listened too much to Greta's opinion and ended up disapproving of Oscar's relationship with Lisa. He was also consumed by Greta's anger that he didn't want her to be disappointed with him and finally followed his wife's way of thinking.
However, he didn't know that Greta's anger was so great that she slapped Lisa. Even though Peter didn't really approve of Oscar's relationship with Lisa, he didn't like it when he saw his wife slapping someone else for no apparent reason.
In the past, Peter was planning to marry Kumala. If the two of them got married, maybe Lisa and Oscar would be siblings.
Until now, Peter doesn't understand why Greta hates Kumala so much and even her daughter, Lisa. In his heart, Peter only loved Greta, and no other woman.