I highly recommend that you start with Carol Werlinich. She is excellent and runs the Marriage and Family therapy program at UMD, and has a private practice in Silver Spring. She is great referral source for family therapy, even if she isn't available.
http://sph.umd.edu/fmsc/people/fac/cwerlinich.html
I would also suggest ask you MIL if she would be willing to pick three therapists that appeal to her and you all try to agree on one together. I would be curious what she says. She can browse profiles on psychologytoday.com.
I won't send you many more referrals as I suspect she will go her own way. Nevertheless, show up for the appointment and determine afterwards if it's going to help improve things.
And to the PP who had the terrible experience as a teen, I am glad you didn't let your negative experience get in the way of trying therapy again later. Unfortunately, many therapists have had no personal therapy themselves and this does a great disservice to the consumer.
I hope your experience in family therapy goes well, OP.