If my Dad left a will, can my stepmom change that will after he passed? Pls read on..Thanks :)

My dad passed away 4yrs ago. After he passed my stepmother gave my 2 sisters, my brother & myself $10 000 each. She also have my stepbrother (her son) $18 000, not sure as to why he received more but anyway.. When she originally told us that we were receiving the money she said she was going to give it to us in one lump sum but then changed her mind repeatedly & actually ended up holding onto mine & my younger sisters (26 at the time) & we had to call her every time we needed some of it. So basically, it felt like she was trying to control us. This was 3yrs ago & I have since stopped speaking to her for numerous reasons. When my Dad passed away she told me that when she passes I am included in the will (I thought I may not have been as myself & my dad had a big falling out when I was younger, which was resolved years before he passed). 6 months later she said that I don't get money but there's money left for my kids, a few months after that she said that neither my kids or myself were getting anything & backwards & forwards for a couple of years. So, what I'm wanting to know is is it possible that she's been changing the will now that he's passed? Or, can she not do that? I don't understand it. Also, originally, all of their joint money/house/expensive items were going to be split in 1/2 in their will with 50% going to myself & my 3 siblings & the other 50% going to my stepbrother. My stepmother did not contribute financially after she married my Dad so it doesn't seem right that my stepbrother (who is not my fathers son) gets 50%... I honestly don't care about receiving any money when she passes, I am just interested to know his it works though. Sorry for my long story & sorry if I've confused anyone... Thanks for the help :)