How to sign low-entropy messages?
Either you make sure that you use separate key (pairs) for each different purposes, or you simply include all possible context in the signature, including the entities.

So sign a message consisting of "Allice: Bob, do you love me? Bob: yes".

Note that it is not required to send all the data together with the signature, as long as you can regenerate the input.



Note that low entropy is not really the issue in your question. It's the amount of context that you sign. Low entropy is mainly a problem for random number generation and the functions that rely on it, such as the key pairs themselves.