Is it considered a sin if I am being requested to have another child for the purpose of giving them up for adoption to a childless relative couple, but I am unable to fulfill this request?
Bismihi ta'ala
No, it is not a sin at all, and I would rather encourage you to be very careful of yourself and such requests.
Nobody can force you to have a child, let alone to give it away. That sounds preposterous.
After you decline, in the case your husband or anyone brings the topic up, or tries to guilt-trap you, inform them very sternly what your view on this is, and if it continues seek consultation.
With prayers for your success