USCIS should have sent the I-130 approval notice to your mother if it was approved. Also, USCIS should have sent the I-130 Receipt Notice to your mother after she filed I-130. So, just check whether you can find the I-130 Approval Notice or Receipt Notice. You also can check the case status online if you have the receit number, which should be indicated in the I-130 Approval or Receipt Notice. Here is the link for the case status check:
https://egov.uscis.gov/casestatus/landing.do
If the I-130 was approved before your mother passed away, you need to ask USCIS to add you as the substitute sponsor to reinstate the I-130 as soon as possible based on the instructions on the websites I cited for you in my previous/above response.
