The lady may be seducing you as per their plan. Else her husband may not have asked you to give her massage when ever she needed. in bathroom also the same thing happend, They have a sex fantasy and you are the other one, (may be due to your age and innocence). The only thing now is how much they will go, will they want sex with you or simply make you jelous. Next occussion when you massage her thigh go liitle up ward, in her inner thighs so as to touch her pussy, If she resist, it means that they wanted you only up to that; that is simply make you fool around.( Dont try anything nasty in future, remain same as you before) if she have no problem, then go ahead, no one usually get this chance in their life, Enjoy it have regular consented sex, There is nothing wrong. As many men want to have sex with young girls, some women prefer young innocent boys.
The main point is that do you want to have sex with her or not, If you want then go a head, else reduce the frequency of your visit to their home, Concentrate on your studies (It should be your first priority), remain as a part of their fantasy, which never fulfilled.
you need not tell this to anybody, Why should you put your relationship in trouble, if you can manage it, as i said above.
Hope this may help you