ester
Not slacking multitasking
The way I see it OP is you either keep the mare, and get help with her (including some physical checks) if your SIL cannot give the right sort of help/encouragement/school her a bit for you then insist that you are able to get someone else in. It does sound like she could be sore/has gone a bit cold backed - I would see if she is better if you at least walk her round, if not lunge first. Do you do much groundwork/long reining with her?
Or if you aren't up for tackling the issue send her back, take a break from the yard and reassess the situation.
Or if you aren't up for tackling the issue send her back, take a break from the yard and reassess the situation.