There is alot wrong with the above perspective you have.
First, you're coming across like you have some power of veto which I assure you that you do not. Next, you're assuming your opinion, if it is unfavorable, will influence your sister's decision to marry the guy, which I highly doubt given how these things go.
None of the above does not mean your heart is not in the right place and that you do not have a right to be concerned. It's perfectly natural to want to get to know the guy. And do you think that can occur over the course of a dinner? Do you think you have the appropriate social standing as her younger brother to interrogate him during said dinner? The answer to the first rhetorical question is no and the second is doubtful.
If you do not know him, unless you live far away, your sister and he are probably rushing into this engagement. However, they have two years to figure that out, and you have that much time to get to know him by his coming around and spending time with your sister and her family - which he should be doing.
No matter how much you feel like you're her protector and the defacto man of the house or father figure, you are not. Your heart is in the right place. Relax. Take a step back...and get to know the guy as you would get to know anyone else - over time, casually. Of course, along the way, if you're asked, you can make your opinions known. But if she has made the decision to get engaged without first seeking your opinion, I assure you that your opinion is not valued in this matter and even if you object, you will have no influence over her decision. That's the cold hard reality.
So chill out...take a step back, and take the time to get to know him and then form your opinion. He asked your mother, she apparently said yes. I'm taking it your mother knows him? Feels comfortable? How does she know him and you do not?