Not a good use of remaining cap space for Buffalo Bills
The biggest issue for the Buffalo Bills right now seems to be cap space and this was probably the biggest hurdle for the team adding DeAndre Hopkins. If the Bills were to add Dalvin Cook, he likely would have to agree to a discounted contract but might be willing if it meant a chance to play with his brother.
Even if the Bills were to agree to a contract with Cook that fits under their current cap space, it wouldn't be a good use of the remaining cap space. The Bills should keep some cap space through the summer in case there is a surprising cut closer to the regular season or they want to add a veteran free agent like Justin Houston.
The other reason it doesn't make sense is if the team adds Dalvin Cook they likely have to release some veterans they added this offseason. Damien Harris agreed to a one-year contract and if cut would result in $1 million in dead cap.
They also signed Latavius Murray after the NFL Draft and if he were to be released that would be over $800,000 in dead cap. If the Bills signed Dalvin Cook both likely would be cut resulting in nearly $2 million in dead cap with almost no savings.