My son was a major sucker until he was 6 so I bought him a pair of those cheep thin knit gloves from the street corner vendor for a dollar and called it a "Magic Glove" I put it on his hand and just like magic he stopped sucking. That is until he removed it, but as it was helping I decided to tape it gently at the wrist ( Clear packing tape works best, not tight just enough so they can't pull it off) and it started staying on at night. after a week I stopped with the tape and the sucking decreased measurably. Whenever we catch him sucking wh have him put it on. I also kept one in his cubby so if he sucked in morning meeting his teacher had a sign for him to get it. Two other suckers ended up getting their own "Magic Gloves" and after a while it became quite fashionable to wear to meeting.
Be careful not to let her replace the oral fixation with substitute...We went through breif phases of sucking on the collar of his shirt, and another genius gave him a chew toy which only excellerated the need to have something in his mouth.
Only use the glove on the sucking hand.