I was reading about the Lux family and thought I might revive this thread. I never knew Maria Teresa's mother-in-law and father-in-law were so mean towards her.

Reading about them has certainly changed my opinion on them. From Wikipedia:
"Shortly after the death of her mother-in-law, Princess Joséphine Charlotte of Belgium, Maria Teresa opened her heart to reveal that her relationship with her husband's mother has never been good because of her Cuban and commoner origin."
"In 1986, Grand Duke Jean renounced the title Prince/Princesse de Bourbon de Parme for himself and his family, when his eldest son, then-Hereditary Grand Duke Henri married Maria-Theresa Mestre. The reason for this was that the Duke of Parma, Carlos Hugo, ruled the marriage unequal in 1981, as well as the marriage of Prince Jean to Hélène Suzanna Vestur in 1987, for which he had renounced his rights to Luxembourg in 1986."
Okay, so not sure GD Jean hated her but it sure seems he wasn't too keen on them marrying and they had to convince him for years. I know Sugars have a lot of info on the whole MT subject (and not all of it seems the usual fluff) and this time I actually think they know something.
Of course no one really has an inside into the family and so nobody besides the immediate family members really know what went on.
First of all, one has to understand that both GDss JC and GD Jean come from a very old set of royalty, both of them were in love with commoners at the time of their wedding but back in the 50's there wasn't even the question of marrying for love. I think it was Queen Margerethe that described GDss JC as "the last true royal" so I have no doubts that GDss JC wasn't thrilled that all of her sons married commoners. Maybe she was a bit more welcoming towards Sibilla because she had a lot of royal ancestry on her mother's side (her grandmother was a Spanish infanta and she is a descendant of Queen Victoria).
Though by all accounts she was the one who welcomed MT into the family and encouraged her son to marry for love. She was the one who convinced her husband and her mother-in-law GD Charlotte to agree to a marriage between Henri and the woman he loved.
Both JC and MT have/had very strong personalities, and by everything I heard I think that it was simply a clash of personalities. JC had very high standards for herself and everyone around her, by a lot of accounts MT did not meet those standards in first years of her marriage. Luxembourg might be a beautiful country but it's tiny and back in the 80's there was even less going on than today. Some people have described it as something like a golden cage for MT and at one point she even tried to "flee" the country; Gui, Felix and Louis were already born at that time. I have heard to different stories: the first one being that she could have left the country but that she would have had to leave her sons behind and the other one being that she made her way to Findel airport and had already booked a flight, then the guards at the airport called the palace and she went back home. The latter one is the story I heard more often, maybe it was a combination of the two, who knows...
If my memory serves me right, the Wikipedia entry must be false cause the encounter with the journalists happened in the early 2000's before JC's death. A lot of people say that the two women rekindled their relationship before JC's passing.
One thing you have to keep in mind that the truth is usually somewhere in the middle: 1) MT is a woman who likes to be the center of attention, 2) she hasn't always cherished her life in the Grand Duchy, 3) I don't think her and Henri's marriage has always been as good as it has been portrayed to the outside, 4) JC is a royal with moral standards few could live up to.
GD Jean might have renounced the de Bourbon de Parme titles in 1986 but the decree is no longer in order. In 1995 the Arrêté grand-ducal governing the names and titles of members of the Grand Ducal Family repealed the 1986 decree. They might not be in the line of succession to the (defunct) throne of Bourbon-Parma, but GD Jean, GD Henri, HGD Guillaume, P Felix, P Louis, Pss Alexandra, P Sebastien, P Guillaume and P Jean are again de Bourbon de Parme's.