It means:
Your sister is with a friend.
Copine can be used in the platonic or non platonic sense, it will depend on context.
With my sister's best friend.
Avec ma soeur
avec ca= with it, this
'je m'entends bien avec ma sœur' means 'I go along well with my sister'in English we would say I get along well with my sister
"je partage ma chambre avec ma (soeur)" means "I share my (bed)room with my (sister)" in French.
You can say "Ma petite amie m'a largué."
'se marier avec ' is 'to get married with ' in English.
with pleasure.
"J'aime sortir avec" = "I like going out with"
Belle soirée avec... means "Beautiful evening with..."
The cast of Danse avec la vie - 1995 includes: Catherine Aymerie as Christine Christian Bujeau as Chef de service Jennifer Chicheportiche as Copine de Vanessa Christine Citti as Professeur de gymnastique Alix de Konopka as Soeur de Philippe Patrick Dupond as Franck Veaujour Didier Flamand as Philippe Jennifer Joubert as Petite fille aux patins Benjamin Labataille as Cousin Vanessa Nathalie Laguens as Copine de Vanessa Charlotte Lasternas as Copine de Vanessa Alexandra Lorska as Professeur de danse Nathalie Villeneuve as Muriel Auclair
Do you mean "C'est avec qui?" It would help to have the context but essentially it means "With who?"