It’s never a nice experience being in a bump, my sympathy to your sister. £100 excess is correct. It’s simply a case of passing on the details to RSA Motability phone line, they will take the circumstances, registration of the other party and any potential witness details.
In terms of third party repairs outside of insurance cover that is entirely at the whim of the other party, however, this does not dictate how your sister proceeds. She is more than within her rights to proceed through her own insurance (RSA) and have them take care of everything, ignoring the other party is advisable if they’re insisting on money for repairs.
Lastly, how did anything seem off? I only ask because the situation seems a little set up. Any connection between the vehicle ahead and the car hit behind? Could be entirely accidental but the swerve in front and reverse is a common scam, typically pressured to go outside of insurance for fast cash settlement. Usually an older vehicle used for the scam too. Anyway, it’s just the £100 and letting RSA do their thing, hope that helps.