ANSWER - It is not uncommon for people to report a connection to a recently departed person (or animal). This can be a visual encounter or something less tangible like in your own experience, a feeling rather than something that can be attributed to the physical senses.
The usual reason for a spirit guide to be sent would be to offer protection to the person they seek out. This doesn’t necessarily suggest anything untoward is about to occur, it could be that the guide has just been sent for quiet strength and solace. It might mean that the person is embarking on a new stage of their life and the spirit guide has come to support them through the transition period. Equally the presence of the spirit guide could suggest they have a message or a warning that they feel it important to impart before fully passing over.
It is interesting to notice that in most discussions about the spirit world the focus is very heavily biased toward trying to find a reason for the spirit visit that links to the person still living. I would say it is just as likely that the spirit has chosen to remain behind/send a guide for their own reasons. They may want closure on an event that they departed in the middle of or they may have curiosity about how people reacted to their passing. I think it is a mistake to always assume altruistic motives on the part of spirits who are surely able to be as self interested as the rest of us.
In the case of your mother in law I think you are probably in the best position to guess her motives as you are the person who knew her when living. If you had an exceptionally close and loving relationship then chances are she is sending her guide for (as above) reasons of help and support. If however she wasn’t that keen on you and didn’t like your relationship with her son then it is more likely she is keeping an eye on you to ensure he isn’t hurt at your hands. Think about what kind of person she was in life and you will be able to make an educated guess from there as to her motives for what she is doing in death.