my sister threatened my baby

i'm posting this in hopes that i can get this off my chest for the one hundredth time because no matter what i do i can't stop thinking about this. i've considered seeing a therapist even though it hasn't affected me negatively just the fact that i think about this every night. so a little back story, last summer my sisters boyfriend of 5 years beat her up very badly, me & my mom were with her in the hospital for 6 hours. the nurse ran out of room on the person diagram of where she had bruisings from him. we knew he hit her prior & tried stepping in, they say you can't help a person who doesn't help themselves which is absolutely correct, but this time she wanted him to go to prison for what he did. which he did! for one month :( also i may add she has an almost 6 year old son who is around all of this mess. after he got sent away she got a protective order. things were great she went to therapy my mother took her & her son in. well this past february she dropped the restraining order & got back together with him. told me & my mom she was going to her friends house for the weekend but had ALL of her stuff packed. so me & my mom followed her walking down the road to see her turn onto another road & get in his vehicle. our hearts dropped. (also may i add they are both drug addicts) so we figured that's why she got back with him because while heing at my moms she kept ripping people off to the point people would go to my moms house looking for her to pay them. OK SO now at the point where i was in labor at the hospital, i told NO ONE. my fiancé & mother were there. the reason we didn't want anyone knowing was because we didn't want my sister there. i had the baby & the first night was perfect. the next day my sister is blowing up my phone & i had told her i was in the hospital but nothing happened yet & i might get sent home. big mistake! she called & had her boyfriend call who she knows we all hate with everything in us. ever since they got back together my family basically cut her off. but anyway the nurses came in & told us she had been calling & we said we dont want her or her bf there. so she ended up coming & they turned her away. my fiancé tried going outside to confront them & they were gone & there was a note on his car that said "i hope your baby gets SIDS you are both pieces of shit" i immediately started bawling with my brand new baby, my world, in my arms looking at her thinking who would say this about such a precious child, any child for that matter. i felt so defeated because i couldn't stop her from saying something like that. it's been three weeks & my heart aches still thinking about it. my sweet child was wished her death at not even 24 hours old. i understand there are far more tragic things & i am thankful my daughter is healthy & happy. but i just can't stop thinking abut what if she gets SIDS. after giving birth which is obviously very stressful & having that put in my head so shortly after & being so exhausted its just stuck in my head & i'm not sure what to do. my sister is dead to me but that doesn't make things better for me. thank you to anyone who took the time to read this, i'm not sure what i'm really asking i just really needed to vent.