need advice.

okay so this may be long.. but really need an unbiased opinion.

my sister has 3 kids and my mom keeps insisting I get custody of my niece who is 10 years old. I wouldn't necessarily say she's "troubled or a bad kid" but my sister gets called to her school at least once a week. if they cant get ahold of her they call me as I'm her emergency contact. my sister just got a new bf and they have a toxic relationship (he is only the father of the smallest kid) my niece doesnt know her dad and I think that may be the reason she acts out? idk... my mom asked me awhile ago if I would take my niece in so my husband and I could "guide her" but I just said I would talk it over with my husband and see what he thought. he said sure, but it all had to be "legal" like my sister give up her rights along with her fathers(who we know nothing about, he is on birth certificate tho) so I didn't feel like going that route I didn't know if we would stir anything up with biological dad and him try to take custody because my sister gave hers up. so we left it as that and never really brought it up again.

my sister is lazy but she does care for her kids, she doesnt work and has her priorities mixed up. she doesnt do drugs or anything, just lazy. kids are safe and aren't abused or anything crazy like that, but my mom just recently again asked me to take my niece in, and I kinda just exploded on her. i told her i didn't want the responsibility and i just couldn't to stop bringing it up because it made me feel guilty and i didn't want to take her in out of guilt. we have been ttc for 10 years and I think my mom just wants my niece to be like a bandaid baby or something. so my mom got all hurt and told me that I was basically being selfish. but I feel as if by saying the truth I'm being opposite if selfish. I love her but I just cant bring myself to fully commit to raising her. my husband said hes on board with whatever I decide but only if it's genuinely from my heart and not because my mother guilted me into it. what should I do? am I being selfish for not giving my niece the opportunity?

add on, answer to questions.

-shes the middle child and my mom just recently took in the oldest which is 13. and the youngest is less than a year old. so that's why it would only be her and not all 3.

-i think my sister would agree honestly, but I think just because it'll be one less worry for her.

-and we do have occasional sleep overs and I'll take her and her older brother to eat or for ice cream.

-i always buy them school clothes and a jacket every winter, because I know my sister cant afford it. I do alot for them but idk why I cant just go 100% and commit to taking her.