You can claim capital allowance on the vehicles but there is a restriction on the amount you can claim. Also, the vehicle must be used in the production of business income.

Disposing the asset within 2 years (i.e. to ownself/family member etc) will cause a claw back to be imposed by way of balancing charge unless there is a valid reason (i.e. cessation of business).

If not mistaken, I think the ownership of the vehicle must be in the company's name and not your sister's name. If your sister uses the car regularly too, she might have to include it in her income tax computation as benefit in kind.