I guess, it depends on each family's situation and personality of each person (kids, father, SM).
I had a baby when my first son was almost 10, so it's almost like having your first child, because the feelings you have for a helpless baby are not the same as you do for older children. At that age, they have many annoying behaviours and might not be nice with you, unless a baby who seems always happy to be with you.
My SD is only 1 year older than my son and used to have really sweet personality when I met her at 7. I used to feel lucky to have her. She played so well with my son and it felt like we were a family. I would take care of her the same way as my son. She was with us EOW and 50% in the summer. Whatever annoying behaviours she had (just like my own son) would be fast forgiven.
When I got pregnant she was already 10 and was happy about it, I was still feeling good even though it was really irritating when she would strongly shake my belly or greet me with "Hi, big belly". When the baby was born, things started slowly changing and I don't know whether it came from me, from her or both. I'm a pretty protective mother, but I don't remember preventing her from playing with him, as long as it was soft (she has trouble controlling her reflexes and compared to boys, plays pretty rough with him even now). At some point, I noticed that she got overinvolved (giving me "advice", telling me what to do with him, getting him out of my arms, doing with him what I specifically asked not to do). Even telling me that it was her baby not mine. I was making huge efforts to be patient, but my irritation just grew and grew. She would just then go home and I would stay with this feeling. My son also did things I didn't like, but I don't retain myself with him as much, because he's my son and it's my job to teach him. AND the irritation is compensated with other nice moments, with "i love yous", etc. SD stopped being affectionate with me really, it went all to the baby. She also asked my DP several times to bring the baby to her mother (who have seen him 3 times), but she wanted more. So I felt that somehow SD was trying to bring my baby closer to her mother, so my link to him bothered her and she's a girl who seems very close to her mother (even sleeps with her at 12), so she could understand. She also started being disrespectful and even told me and my son in different moments that she would forget about us if/when I break up with her dad. And we never had a fight, it was just comments here and there.
So all this to say that in my case, being a SM became much harder. I felt like I wasn't important to her despite affection and care of the previous years and that removed my motivation about her. I still do what's right, but inside me I know that nothing replaces your own children. No matter what hurtful thing my own son can say in a heat of the moment, I know that he loves me. With SD, I don't know. I actually have no idea what she feels for me at this point. My son is 17 months now, SD stopped being openly disrespectful (I think because my DP finally talked to her), but inside me, I still can't get over her behaviour....