Yeah I mean sure, why should it matter if your older sister left her former lover in order to take care of you. Steal him away from her anyway, seems like a totally justifiable thing to do and doesn’t make you an asshole at all.
On the flipside, maybe it doesn’t, because the sister in all this time never even bothered to contact him or try to follow up and get back together with him.
In short, they’re both horrible people!