How hard is it to for a Romero motion to be granted?

You can not do anything about your doubts regarding the attorney because you brother is the client not you. Beyond that a Romero motion is always difficult to win but not impossible. It will depend on facts and circumstances which we don't know here in hyperspace. If you have questions about what you can do to help you should put aside your distrust and ask your brother's attorney.

A family can help in a romero by writing letters to the court and by being present for sentencing. Often times I will ask that family members assist by helping to show that the client has good "future prospects" and by having them do the legwork of tracking down resources that are available upon his release. For example, helping him get set up with a job or getting into a drug program upon his release etc. You should discuss these matters with the attorney and express your concerns. If you don't get the answers your need then discuss changing attorneys with your brother.

Hopefully your brother has more faith in his attorney than you do. The concerns with respect to the Romero motion (to strike the prior strike, res. burg.) are the facts that the res. burg. was only two years ago and presumably involved similar conduct of theft. If your brother is dissatisfied with his attorney, he and/or you should consider hiring another attorney for sentencing and/or considering to attempt to withdraw his plea. But be careful what you wish for.