Someone's being a tsundere here, she's jealous of his sister knowing that she have brother herself and wasn't treated the same way, nu uh, you can't hide that brocon stuff from me.
She also clearly sees a reflection of her brother on Izumi which will probably lead to comedic "im not a shotacon!" and "w-we are family, this thoughts are wrong" moments.
A brocon aunt who can't let go of the past (losing to 'that woman'), intruding upon her nephew's and niece's personal life, and noting that said nephew represents his dad (AKA her onii-chan)...
Where have I seen that character before?