I suggest your sister-in-law reapplies for PIP.
This time do it differently. If you see the PIP points table: https://www.citizensadvice.org.uk/Global/Migrated_Documents/adviceguide/pip-9-table-of-activities-descriptors-and-points.pdf
She should think about each of the activities in turn, consider which descriptor fits her best. But she needs to consider that she should be able to do each activity reliably, repeatedly and safely. This means for eg, she might be OK to walk 50 metres, but does she stumble, or fear falling, can she do it again immediately or need to rest, does she stagger? She should do the same honestly and carefully with each of the Daily Living Descriptors too.
Then add up the points total. If she scores at least 8 by her own honest evaluation (or does it together with a good friend who can help her evaluate her capabilities), for Daily Living and/or Mobility, then she should reapply.
But, she needs to be able to a) write the information down in such a way that the assessor understands her capability, and b) prove it by means of written evidence. This could take the form of letters from neurologist, MS nurse, physiotherapist, OT, anyone who can write a letter and state exactly how she meets the criteria. She could also attach photos of aids and adaptations within her home, showing how she manages to live.
She should approach it by thinking in terms of helping the assessor understand her MS and her life. So not assuming that the assessor is an adversary, but instead like they are working cooperatively to work out where she fits in the PIP table.
For help with this, she could get assistance from the CAB, or look at their guidance: Personal Independence Payment - Citizens Advice or consider joining https://www.benefitsandwork.co.uk/ Their guides cost money, you have to become a member and that costs about £20 per year, but they are excellent and very helpful.
Best of luck to her.