For the fantasy, give her the brits. She likes knights, give her that, or skaven. Both can be allied with other races easily and she can expand from there.
40K might be a bit of a problem as she needs to read many codexes to know the basics, but get the online version and print the army list for her. As said, the Tau, IG, GK, SOB, Eldar, and SM.
She will have fun with this army. SM goes shooty, SOB becomes meat shield, GK deep strike, Tau crisis suits jumping here and there, IG troops in chimera and Kaserkins getting in close diverting enemy shots while harlequins dances here and there whipping the enemy up with their Initiatives
Do you reliase 40 SOB troops only cost less than 500 points with 2 meltaguns, and 2 storm bolters and with krak grenades. That is a serious meat shield with with enough bolter fire and numbers to make your opponent hesitate to assualt and attack tanks if your sister wants. Each squad consist of 1 special weapon and 1 other weapon. Each model cost 11 points and they come in plastic.
Mix and match the good guys, this she can decide. But if you are making an army for her, and you have many unused models, this can lower your spending cost. Get her a normal SM HQ and 2 troops, than start mixing from there.