First thing first, please tell her to get them out of the fish tank. There is not proper ventilation in there for one no matter how big it is much less for two.
Poor things. You can either go out and spend huge amounts of money on the biggest store bought cage there is or go to cages
and make one yourself. Trust me I made my own without help from my husband for that matter and it was simple.
Onto pecking orders. How old was the guinea your sister already has? If that one is the older one, yes, most likely it is a display of dominance. Putting the smaller in its place.
Sexing can be done at source
also, try putting some pressure right above the "privates" and push in. If something pops out, definately a boy, if nothing pops out it's a girl.
There is always the possibility of them never making out their differences, and in that case she would have to house them seperately.
Anything else I can help out with let me know.