I get on well with my exhusband, it's like a brother/sister relationship. He has stayed at my house with his 2nd wife because I live next door to DD2 and I have the spare bedrooms. In saying that I did feel uncomfortable in years gone by when I found out he was at my DF's house with his 2nd wife. I am happy to spend family occasions with him and his wife and my DC and DGC as it makes it easier for my DC. My DB is getting married next week and I told him not to invite my ex, that would feel uncomfortable for me. My DB had asked me as he had been in contact with my ex for business reasons. He has known my ex since he was 12.
Recently I was talking to my exH's step DIL at my GD's birthday party, she told me next time I am back in our home city to get my DD to let her know and she will arrange for us to come for dinner. I said that would be lovely, but I wouldn't do that. There is a fine line with exes and you shouldn't cross it, especially if it isn't your ex.