Can my mother apply for me again if she is a green card holder?

Your mother can also apply for you as long as you remain unmarried until you get your residency. How long it will take depends on what country you were born in.

Your mother may file an I-130 for you. Doing so will not impact the pending I-130 filed by your brother. You can review the Department of State Visa Bulletin to compare the wait times for priority dates to become current. You may want to look into employment, student, or investment based visa options for the time spent waiting for an available priority date.

You are able to have petition from your brother and mother. It will take many year waiting with a petition from your mother.

IF your mother applies for you as a green card holder, depending on what your country of birth is, it can be anywhere from 8-20 years before you qualify for an immigrant visa; if you wait for your brother's petition, it will be 12-23 years, again based on your country of birth.

Your mother, as a lawful permanent resident, is entitled to file an immigrant petition (I-130) on your behalf as an F2B "Unmarried Daughter (21 years of age or older) of a Permanent Resident" even though your US Citizen brother has already filed an I-130 on your behalf as an F4 "Brothers and Sisters of Adult US Citizens" since, in some cases, the waiting period in the F2B classification may be shorter than the waiting period in the F4 classification. For example, processing times under the USDOS June Visa Bulletin indicate that a Beneficiary from a chargeability area that is not China-Mainland born, Indian, Mexican or Philippine would have almost three and a half years less wait for an immigrant visa process in F2B (15 April 2004 priority date currently being processed) as opposed to F4 (08 January 2001) if both I-130's had been filed at the same time. The opposite would apply, however, if, for example, you are a Mexican national since, currently under that national chargeability area, an F4 has a several year advantage (01 June 1996 priority date) versus an F2B (01 January 1992 priority date). Unfortunately, under either scenario, you should expect a significant delay in years before either petition will actually generate an available immigrant visa slot for you as the Beneficiary under our present highly-restrictive and "broken" immigration regulations, unless the US Congress decides to increase the number of available visas in the various categories at some time in the future.

Yes you can file another application through your mother. It will be shorter time if you file a family petition through your mother. Once your visa is available, you would have to withdraw the other pending application; the one filed through your brother in this case.