Your mother becomes eligible for an immigration visa when her Priority Date becomes current in Table A Final Action Date of the Visa Bulletin. https://travel.state.gov/content/travel/en/legal/visa-law0/visa-bulletin.html
If you look at the latest VB for October 2020, the Final Action Date for the Philippines is August 1, 2011.
It seems that it took USCIS about a year to approve the I-130 and forward it to the NVC. This means under the Child Status Protection Act (CSPA), you and your sister have about an extra year after turning 21 before you age out and are no longer eligible derivative beneficiaries. After age 22, you will not be able to immigrate with your mom.
It seems that it may be another 2.5 to 3 years before your mom's PD becomes current. So, it's extremely close for you. You may age out. Your sister may age out too. There is no way to know if either of you will age out or not. You have to just be patience and wait.
Your aunt got her letter for further processing based on Table B of the Visa Bulletin. EVEN IF you and your sister are eligible under Table B to apply for visas, it does not mean that you are actually eligible for the visas. Ultimately, eligibility for visas depends on Table A.
I am surprised that your aunt's case is being processed by the NVC since a PD in 2013 in the F2b category for the Philippines is not eligible for further processing according to the current Visa Bulletin.
You and your sister will get some cspa protection, which may or may not prevent you aging out. Do you know the date when the petition was approved? (Your grandfather would have received an i797 approval notice with this date some years after the petition was filed originally.) It actually has nothing to do with the date of the welcome letter. The important dates are your birth dates, the priority date, the approval date, and the date that a visa eventually becomes available.
