A few people further up have mentioned that they think it is bad that Isabelle has to lie about loving having sleep overs with Jace and for that matter, the fact that she is roped into sister sleep overs with the younger girls.
I think it is worse than that!
I think she has lived such an isolated, emotionally stunted life where she has been gas lit so much, that she actually does enjoy these things,and many more bizarre things, which are not normal for a girl her age.
Let's not forget her highly inappropriate and sinister reaction when Sarah told her she'd stopped breast feeding Jace.
I agree, there is something very fucking not right but in this issue, lying is the least of it.