TLDR: exSIL is exH's sister and lives abroad. After we split I made a huge effort to take the DC to see her and her family when they came to the UK, invited them to dinner etc. SIL was nice to my face and then slagged me off to all and sundry. Someone told me, I called her out on it, she threw a tantrum and I haven't heard from her since. This was ~5 years ago.
In the last few months she's started sending cards to me and the kids. Two cards arrived this morning. WIBU to write back telling her not to contact me and to send anything for the DC to exH's address?