I wouldn't worry about it, whether I were you or your mother-in-law. Both the Pope and the Ecumenical Patriarch have officially proclaimed that the Roman Catholic and Eastern Orthodox churches are sister churches. It doesn't matter which one you're in, you'll be saved in either one.http://www.youtube.com/watch?v=7XiHOUg1UqM&feature=share
Forward to about 29 minutes in to see the Patriarch proclaiming this teaching.
Of course, you might suspect that what the Pope and the Patriarch are now teaching does not constitute traditional Orthodox (or Catholic) ecclesiology. In that case, I think I might be able to help you.
Btw, the more traditional Catholic ecclesiology might hold that Orthodox sacraments are "valid", but because we are in schism, the reception of these sacraments is only to our condemnation.
While I respect JG's knowledge and often find myself in agreement with many of his posts on faith issues, JG and I disagree on this issue as to what the actual position of the canonical Church is on the matter of Orthodox relations with the Church of Rome. I have previously posted and sent several of you the links from SCOBA and the Consultation which refute what his Church claims in their video through thirty years' worth of published joint papers on Orthodox Catholic relations which clearly outline the large areas of doctrine in which we concur and the more important areas of doctrine and organization in which we remain far apart. These issues have been argued 'ad naseum' in other threads over the years and it is more important for a 'newbie' - especially one coming to us from the Eastern Catholic side of the aisle, to focus on her own faith journey into Orthodoxy and worry less about what her relatives may say or upon the sad, but real divisions within our own home.
I suppose it doesn't occur to you that the official "Church" talks out of both sides of its mouth (like many other heretics before them, such as Arius and Eutyches)? If the Patriarch says something heretical one day, as you see plenty of times on that video, but some other day says something resembling Orthodoxy, does the heresy he preached the former time not "count"? I would be more impressed if Pat Bartholomew simply renounced all the heretical statements about "sister Churches", but he never does. Unless you can produce some written or video evidence that he has done so.
But actually I think it's important for a newbie to know that the actual position of the official hierarchs is not Orthodox. I understand that she's on a journey into Orthodoxy, but why should it be hidden from her that the official position of the official Orthodox churches now is that it's not necessary to be a member of the Eastern Orthodox Church to be saved? The EP and the MP say the mysteries of the RC are valid, the AP that the mysteries of the non-Chalcedonians are valid. If she is worried perhaps about being outside the Church because of what her MIL has said, I think she's right to take that seriously: it IS important to know where the True Church is, and whether one is in or out of it. There's no getting around this concern by saying that it doesn't matter, because it DOES matter.