Step child attachment

I am 23 and my S/O is 33 with a 7 year old daughter from his first marriage. She is amazing. She has been extremely attached to me since meeting. She wants to lay with my on the couch, constantly hold hands, she even comes down in the morning to get in bed and snuggle with us. I don’t mind at all however he has made multiple comments about her attachment to me being much greater than even the way she acts with her mom sometimes. She even tends to favor me over him often times. We thought this would fade some once I wasn’t “new and exciting” anymore but it’s hasn’t at all. I haven’t done anything to promote this or stop it.... I just let her act how she feels comfortable and be her friend. She has even made comments to me about wishing I was her “mom” sometimes.

Has anyone else experienced anything like this? I know her situation at her moms house is less stable than with my S/O and I (he has primary custody of her) due to altercations during their marriage.