1) If someone says Wallah for something and they forget are you obligated to remind them.

2) Do they get sins if they forget? And if I remind them with that then they will not get sins

1) You are not obliged to remind them. It’s on them to remember.

2) Yes if they do not go through with an oath it is haraam. You can remind them if you like.

However if they were not serious when they said Wallah, they will not be held accountable for it.