You're not really breaking the game but like the game tells you in the beginning, the love route that leads you into having a harem is the main route at the moment. So while you will be able to go for single girls later down the road, for now you need Sandra to at least get all the household girls. Without Sandra you won't get Lucy at the moment. If you just want to go for Sandy and Lucy however, that works. But you need Sandra.
It is a harem game. No one will get mad if you go for every girl in the game, this includes Lucy. Lucy won't be angry if you have other girlfriends
.