It's amazing how such a simple thing can be involved with so much truth (or lack thereof). People can believe what your mother-in-law does for many reasons, such as having been hurt, not believing in exegesis, or because God
actually wants her to. One could investigate whether her conviction from God or something else (if she were willing).
It's never our job to tell someone what they ought to believe. If they are willing to listen, we can share what we believe and why. Rather, since this is not a core issue for a right relationship with Jesus Christ, we can be loving, supportive, and encouraging. These would have a far more beneficial effect on her than resisting her on a disputable matter. (Romans 14:1) This is even sometimes even the case when what she believes
is a core matter and she is sinning from your perspective. God himself does not expect us to stop all sinning all at once (because we can't), but rather tends to work on us one or a few issues at a time. Imagine what it would be like if someone pointed out all the sinful things you were doing expecting you to stop them all immediately. It would be extremely oppressive (and not from God).
God sometimes wants people to be faithful in certain things that are not Scriptural commands because it is something they
can do. Their faithfulness to God is what is important and they will be rewarded for it. I can't think of a single example in Scripture of God rebuking or correcting someone for something they thought was a way of being faithful to Him. Rather He meets them at their point of need and points them in the right direction (when they're already trying to head toward God).
We can always pray for them regularly. However, sometimes it is our perspective that God ends up changing.
