If you just need an LTL expression for 'If I love Masha, I can not love Dasha' then this might be enough:
bool i_love_masha;
bool i_love_dasha;
ltl la { i_love_masha -> !i_love_dasha }
then the question is what is the behavior of the model. I'll guess something like:
init {
i_love_masha = true;
i_love_dasha = true; /* should be a violation! */
}
Maybe that gets you started. But, I'm not sure if this exactly answers your question!