I'm going to go against what everyone else seems to be saying. I do not think anything has happened to your DD.
Reading your OP, it is your MIL who has had her alarm bell go off, and only because your DD did something twice, during one bath. You, the mama bear, have not had your alarm go off. You have not seen anything else to indicate that something bad could have happened. All you are reacting to is your MIL's alarm, which you mentioned may be overly sensitive do to her own abusive past
I will tell you that my 4 year old DD became obsessed with genitals after her brother was born. She tried to touch him all the time. It made me nuts. Then, she decided it was funny to try to get him to touch her. She would do things like stick her butt out to see if he would spank her. I can completely see her thinking it was funny to grab a 2 year old brother's hand and try to get him to touch her. His protests would just be part of the game.
I understand you being on alert. I think that is what a good parent does. But if you personally are not seeing any changes in behavior, or seeing any red flags, than I think taking your child for physical exams of her genitals or taking her to a therapist may be over the top and could be scary and damaging in and of themselves.
On the other hand, if you do have a mama bear gut feeling that something is wrong, and MIL's suspicions simply gave those feelings a voice, than by all means ignore everything I just said and get your DD some help.