Is this NTR as in MC is cuckold or do we do the stealing. They should really create a different tag for this type of issue because cheating is not the same as NTR.
There is NTR here from the MC's perspective - Eric takes the MC's Mom and the MC can either support him, try to string him along or go to war with him. Then Eric begins to work on the other women in the house, and the MC will be warned not to interfere. No matter what he does, Eric will start to make recommendations that his Mom will follow regarding his sisters and these will, in turn, limit the MC's chances to do things with his sisters. Finally, Eric is dropping hints and starting to put pressure on the MC's Mom that the MC needs to leave and go to military school to become a man and the MC is not going to have a say in this.
Add to this that the criticisms of Eric that we cannot fix since the MC is being forced down a Beta path.
The creator could do the following:
Give the player the ability to work on becoming an Alpha - exercise, education, persuasion skill that can actually be used on key decisions to Stop Eric.
If the MC can buy clothing and candy for his sisters, why can't he buy barbells and start to work out in his room, for example.
So far, the exercise skill is not being implemented, there is no education skill and persuasion has no impact on Eric's attacks on the MC, not even with his Mother.