Firstly, Sirius Black was a famous murderer, and it is a bit of a stretch to say they wouldn’t recognize him.
Sirius Black was a murderer when Fred and George were likely to be toddlers. So they wouldn’t remember from the time. The only way they would know would be from hype afterwards, or from Daily Prophet articles etc. It is likely that they wouldn’t have known the story as detailed as we do, and didn’t know the name of his victim. Especially as he was only one of many victims. Note, Fred and George were unlikely to sit and read newspapers, nor were they likely to listen to Molly’s reminiscing. So where would they have found out about it?