Is it normal for a child with global delay to masturbate?
Is it normal to masturbate with your sister?
Is it normal to masturbate with your brother?
It's ok but some societies frown upon it. As long as you both are enjoying it keep doing it as long as it legal. Do not touch each other as that becomes Incestuous behaviour which is illegal.
Is this normal found a pair of your underwear in your younger brothers room with cum stains on them I know boys have fantasies with panties but is this normal to masturbate to your sister's underwear?
If it was normal, then your brother would have his OWN pair of underwear, not panties, but still.