If you are traveling with her then its no problem at all.
If you are NOT traveling with her then:
For checking in it is usually not a big deal;
For the security line it may be an issue and for the boarding it is more likely to be a NO. I would suggest you ask the check in person (very nicely) for a gate pass to walk your frail mother in law (make sure she looks the part) to the gate and they usually say yes; then you could flash your elite card (and I assume you are elite) in security and then she could board earlier (because she is frail and needs more time);
You could also request for a wheelchair to take her to the gates and that usually gets you the VIP treatment.