so I need advice I'm new here

So recently my dad got diagnosed with terminal cancer and is currently going through chemotherapy but the chemo is a comfort only thing at this point. Anyways I find myself super angry because I want my dad to walk me down the aisle I've been engaged 4 years Christmas 2016 and we'll be together 5 years in May of 2017. My sister and my brother (different mom's same dad) got to have him there at their weddings. I'm feeling jipped because I want my dad there so badly. I'm so angry at my sister in particular. I'm angry and resentful all around any advice?