Walburga may have hated Sirius but she still loved him. He was a problem child and needed to be disciplined, but she saw herself in him and only wanted the best for him in her own fucked up way
She didn’t love Regulus not even one bit. She never felt maternal towards him. He always listened without second, too quiet, he was too moldable, too blank to love. She only ever saw him as a pawn.