After your brother returned you felt wierd and you felt a cramping as you looked down to see changes happening.
You realise that your hips felt weird. When you look down you were shocked to discover- Your legs are black and shiny?! Then you feel large amounts of pressure on your athletic legs as your pants rip!
Six more legs rip out of your pants completely destroying them, and they look like... a scorpions leg?
Now your original legs are starting to look like your others.
Then your hear a loud RIIIIP!
A shiny appendage is bursting from the back of your pants!
It grows into a full tail, complete with a stinger. You realize you have the lower body is now that of a scorpion! Then you notice a large amount of Chitin spreading! Another pair of arms burst from your shirt! Finally, your eyes change color -red with black pupils- And you grow flexible "fangs". You look around, disoriented, and flex your tail.
Your brother watched you as your lower body grew larger and your upper body grew in proportion and you grew fangs in place of your teeth and the armour plating of your lower body became finer the closer it came to your breasts which grew into EE cup. You looked yourself over as a 7'10" tall anthro-scorpion with a proportionally large lower body and torso you look at you brother