It's not Welsh. It sounds a teensy bit Welsh because the ending - wyn is a common element in Welsh names, from gwen meaning 'white, fair, holy'. -Wyn is the male form, -wen the female form. Shell doesn't exist in Welsh. 'SH' doesn't exist in Welsh for that matter. And there's no word or element in Welsh that resembles 'shell' phonetically or visually.
Your mom made it up. Evidently a few other people's moms did too. If it were me, I'd write them and say hello :o)