I am a beneficiary of the RSA. Am I still required to settle my sister's inheritance tax, of which I am the sole heir?

There is no exemption from inheritance tax for beneficiaries of the RSA.

You can negotiate payment terms with the tax administration or ask to benefit from the split payment. In this case the duties shall be paid in equal installments, the first of which shall be made at the same time as the declaration of succession and the last one not later than one year after the expiry of the statutory time limit for subscribing to the declaration. The number of installments is three. The interval between two payments may not exceed six months.

The deadline is extended to three years and the number of payments to seven when the estate assets comprise at least 50% of the illiquid assets (real estate, antiques, art or collection ...).