If you think your mother in law would like to come and you get along with her and you feel the techs will be okay with all the people being there then I would say to invite her.
It sounds like that is what you are doing
I have two related questions (since OP already had gotten the advice she wnted and made a decision I hope she doesn't mind if I ask):
1. Maybe this is an age thing (my kids are already 12 and 14) but when mine were born the standard was just one ultra sound at about 20 weeks. Has this changed?
2. Another possible age thing: Do most women have their husbands at most Dr. Appts for pregnancy and do most people have a "crowd" at the ultrasound? There was another thread recently in which family was coming in from out of town to see the ultrasound. My husband only went to two appointments: the ultra sound and one late on after a move when I was having trouble communicating with the new doctor and wanted his help. Generally, it never occured to me that i wanted or needed him at routine appointments--gosh save up the good will to miss work for when the baby is here

and it certainly never occurred to either of us to invite people to the ultrasound (we have never been invited to one either).
Is this a regional thing or an age thing or what