Humping problem

WELL, I have two pretty young dog's(2 and 2 1/2), both female's and have only lived with each other for a year. My older one humps the younger one. Prickle(the older one) is the more dominant one then Bro(the younger one). Bro is bigger then Prickle as well. They are BOTH desexed Prickle at 4 months and Bro at a little younger then a year old. Prickle will hump bro anytime but only at home and Bro is the only dog she does it to. Any idea's why she does this? I don't mind them doing it, it's not hurting anyone and Bro doesn't care.

Dominance posturing. It has nothing to do with sex.

The older one is just showing her dominance. My female dog has been humping dogs since she was a year old. She is showing she's the "alpha female"

Yep, it's purely a dominance thing at this point. Not an issue with the dogs, as one will always be the dominant one and it's better to have it showing this way than by fighting, but if it starts happening with people you will need to start reprimanding for it.

You can try discouraging it and maybe spray with water in spray bottle. Medically, as a registered vet tech with small animal clinic, there is no deffinition. They just do it. :/