@bettybob
I don't know why you can't take her to lunch and tell her you love her like a daughter, which is why you are going to talk to her like one. Also tell her that you want to help her help your son. Then tell her that you feel like she is always putting you down. Ask her if you are reading it wrong.

Maybe lead in with, "i hope i don't seem insecure about this, but I..." or "i hope you forgive me for asking, but I....". Being direct is the best way. She is probably the one who is insecure.

If you dont really love her like a daughter, then it won't matter if she gets insulted, or you can tell her anyway for etiquette sake. We only have one life. Best to know where you stand than fret about something that might all just be in your head and not hers. She mightbe defensive because maybe she thinks you are the one who doesn't like her. She also could just be psychotic.

[FYI choose a place you really like if you need a little emotional support, as if you are sharing a part of yourself. You can small talk about why you like it before you lower the boom...or her favorite place and ask her why (see the world from her eyes)...or what i would do is ask her what place she has always wanted to try so that you can create a new experience together. You can small talk about what she thinks about now that she's there, and then lower the boom. Preferably during the dessert course so that you can either finish quickly and leave if it goes bad or linger over coffee or another wine if it goes well. Also, take separate cars/cab if driving to avoid a potential very awkward ride home. That will take the pressure off of you to play fake nice and not say what you really want to say. A real heart to heart requires being vulnerable and supportive.

Or she might really be a terrible person and soon to be divorced from your family, in which case you can have a deliciously nice lunch watching her squirm.

Have fun. Let us know how it goes!