well i would just ignore it, eventually it will get in the past. Depends how your mother usually is (does she question things a lot, or does she let it slide if you don't say anything?) Mine would probably let it slide, considering it's an awkward topic... Or maybe she would ask ab it, in case she asks u can just say that you found it on twitter randomly and out of curiosity you glanced through it. Or if you wanna clear it up, go up to her and say "idk if u saw what was on my phone,,, i found is online accidentally etc etc"
i guess saying the truth is not an option haha good luck - anyway she may not have seen it!