Does advising the sale of something instead of giving it as charity prevent one from receiving the reward of charity, knowing that I continuously assist this relative financially?

If your sister has a real need, and you prevent her from giving charity to start with her own needs, which she cannot fulfill, then there is no blame upon you for that. In fact, you may be rewarded for it, because she has been commanded with what aligns with the Legislator's intent: prioritizing one's own sustenance, then that of one's family, then one's relatives. The Prophet, peace and blessings be upon him, encouraged starting with oneself then one's family, and he forbade squandering wealth, unless the giver of charity is known for patience and prioritizes others over themselves even if they are in need.