My mother in law has my son once a week, sometimes its once every 9 days. And she has him for the day.
However she hides the fact she is having him from her other son who also has kids even though she sees them most days.
Yesterday we dropped him for the day but at 3pm we got a call asking if we could pick him up cause she has a doctors appoibtment.
Anyway it turns out she did not have a doctor appointment…she went to her other sons to see their kids…again and did not want them to know she had our son.
By the way, as to your question, what would I do in your situation? I think that you should have a conversation directly with your mother-in-law about what it means that she’s lying. You should inquire as to why she’s hiding things about your child from the rest of your family. It sounds insulting and it probably is when it sounds that way. Hopefully she’s cool enough that she can just have a conversation with you directly if you initiate one