I'm going to yell at my sister for not seeing our imprisoned elderly father on his birthday
How do you feel about your sister?
She's my sister and I love her deeply
If you love her so much you need to respect her decisions.  Kids are allowed to cut off contact with their parents, especially after their parent has done something bad enough to land them in prison.
I just want to show her love and how much I care about him. I think she should show her love for a old person and have spent time with him.
You cannot control what other people do and this is her decision. It is best to respect this decision so you can keep your relationship with your sister.