How old are the children? If they're teens and/or adults perhaps you can speak to them? I would try to mention little of the fallout between you and your sister other than the fact it occured (to remove all other intentions other than wanting to maintain your relationship with them), which could serve as an explanation to your future absence. You could maybe let them know in spite of the strained relationship with you and their mother, that they're always free to contact you about anything? I don't know. Just something that shows you care and want to maintain a relationship with them. After that, it's up to them to decide whether they want to do the same.