Am I obligated to fulfill a vow I don't remember, which involves sacrificing a sheep monthly, knowing that my husband has made a similar vow?

If you suspect that you vowed to alternate with your husband in slaughtering a sheep, then caution is obligatory, and you must fulfill what you are doubtful about until you are absolved. Caution in religious matters means fleeing from doubt to certainty. You should discuss this matter with your husband.

If your husband vowed to slaughter a sheep every month and accepted that, it is more cautious for him to intend in one of the two months that he is slaughtering the sheep on behalf of the one of you for whom the slaughter is obligatory, and you can assist him. However, if he refuses to slaughter every month, it is more cautious for you to slaughter in one of the two months to ensure that you fulfill your vow.