Actually, I was going to agree with everyone else at first, but after thinking about it I’m not convinced that making someone forget something would be overstepping, it’s pretty close but it’s not necessarily an unreasonable suggestion. A person can absolutely make themselves forget something, ever heard of a suppressed memory? A better example is hypnotism where the hypnotist tells their “victim” upon awakening you won’t remember any of this and then they don’t. I frequently compare the suggestion spell to being hypnotized but being magical it’s faster and has a better chance of working.
Now with hypnotism it doesn’t always work, many people still remember what happened after “waking” from a suggestive state, and those that don’t usually regain their memory over a period of time, usually within a few hours if not a few minutes of being released from it. Also, it suppressing anything that affected a large span of time in a person’s life, like that they had a brother or sister, would fail, but I could see it temporarily causing them to forget something that just happened.
Naturally when the suggestion spell ended they’d remember again and act accordingly, but I think whether or not it works may be dependent on the GM’s discretion and what exactly you asked them to forget. Personally, as long as it was a recent event “just happened” and not terribly traumatic I think I’d allow it. Again, with the caveat that no matter how you worded it would not be permanent, it’d just last until the spells effect ended, so 1 hour / caster level.