I am so glad that you have reached out here today, welcome to this really supportive community in what it such an overwhelming and extremely emotional time. I am so very sorry that you have lost your nephew and I wish to offer you my heartfelt condolences at this time. There are a few different issues going on here so I have no idea at how emotional and how pained you are, dealing with guilt and grief and sadness all together.
I totally understand the pain that you have felt from your sister's text, you were trying to reach out to her at a time when you wanted to give your support and love and she has seen it as an attempt for you to use her pain as an avenue to reconnect. Can I say that your sister is going to be experiencing so many different emotions too and that her lashing out at you is potentially apart of her grieving. Not only has she lost her son, which she will be devastated about but when a parent losses a child to suicide there is a whole other layer of pain, which includes blame, at herself perhaps, at her partner, potentially at you, she has to get through a few hurdles to be able to know that she is not to blame and nor is anyone, no one is to blame for suicide, and I hope that she can get some help and counselling to help her through this time.
That doesn't change the fact that what she said was hurtful but she is in so much pain now that her lashing out is almost expected. Please do not take what she has said to heart, while she still may not want to repair the relationship with you I am certainly sure that she does not think you would be happy about his death.
I don't think you would have made her grief worse, I don't think that there is anything in this life more painful that a parent having to bury their child due to suicide, so she is hurting beyond words right now.
I think what you can do it perhaps write her a card or a letter, this is just a suggestion by the way, explaining that in no way did you mean to hurt or pain her anymore that she is already and that you were simply reaching out to your sister, in her time of need, forgetting what has happened in the past and offering a hand to hold. That you are most certainly not trying to address the relationship between you and her, that if she doesn't want to address that it is fine, but you are putting your arm out to hold your sister at this most horrific time.
I am so very very sorry that your family is going through this, we are here for you.