This might sound obvious, but is the reason Sans clings to Aliza so much is because she looks like Frisk?
Partially. He also clings because she’s his daughter, his baby girl, his precious angel. One half of his only surviving family, and all he has left of his wife. To lose her would be the end of him. So, he tries to be a good and fair parent… but he’s terrified to let her out of his sight.
Last time he let someone he loved go, they never came back. You understand his hesitance.