TLDR; Euippe is Diomedes' wife in Italy and probably maybe Turnus' sister
She's mentioned in only one source, in the case of Alexandria with Alaeus and him claiming her and pissing Diomedes off ending with Diomedes cursing his land over it. A lot of mentions of Diomedes ending up with a daughter of Daunus who is often unnamed but given that we have one mention of Euippe's name our best bet is assuming it's her lol, that's where we get her probably being Turnus' sister from. There's also a mention of Diomedes and Euippe having two children!
Euippe's name means good mare which I think is very cute
The assumed storyline (cause we only have mentions and bits and pieces) is that after Diomedes' exile following the Trojan War, he ends up in Daunia, Italy (modern Apulia) where King Daunus begs him to help fend off the Messapians. For his assistance, he hands his daughter Euippe's hand in marriage to him. She goes on to mother his kids, Diomedes (jr.) and Amphinomous.