My husband gets mad if his 8 year old doesn't get everything my 11 year old does: Advice?

I am looking for advice on a family issue. Background, I have an 11-year-old daughter from my first marriage, and my husband has an eight-year-old daughter from his. Together we have a four-year-old daughter. My husband drives me crazy because my 11 years old can’t do anything without either the eight-year-old doing it as well or my husband getting super pissed off. My 11-year-old has a sleepover with a friend on the weekend— my husband will flip out if the 8-year-old can’t do the same. My 11 year old finally matured to the point where we gave her a phone (very, very monitored and limited use), and now my husband won’t stop talking about how the 8-year-old needs a phone. I told him that she’s not ready for one yet, just like the oldest wasn’t at eight, and every single time, he snaps back with, “well, if her sister has one, then it’s only fair if she gets one”— y’all it’s exhausting. It is with everything. My 11-year-old can’t ever have anything special or to herself. It makes her mad, and I can’t say I blame her. Is there any way to make him see that he’s being a little ridiculous, or is it just a lost cause?