All I know is, whatever you do is wrong with grown up children.I have always put my family first to the expense of friendships. Now I rarely see the grown ups they are, they have both moved on.Tigergranny, I always looked after cards and presents for both sides of the family as didn't want anyone left out as they would have been if my husband was in charge of that side of things. He was a wonderful generous man, but forgot birthdays and dates, but never my birthday or wedding anniversary, I miss him so much and it certainly never bothered me one bit that I did it, I just used to tell him what he had sent them, get him to sign the card so his parents thought he had bought it. It meant such a lot to them.
Roxy my father remarried after my mom died, my sisters never forgave him and didn't see him in the end. It didn't bother me him remarrying, as I wanted him to be happy and he and mom had a good marriage and he missed the companionship. People who have never known lonliness cannot understand how bad it is. So make his children welcome if they visit, remember special dates, you can't and don't want to replace their mom, hopefully they will accept you as their father partner who makes him happy. Is your friend on her own and perhaps a little envious, she is probably missing the closeness you had and if you hang on in there perhaps she will accept the new different way your friendship is now as you have your partner to do a lot of the things you did with her now. Just don't make yourself unhappy because of it, life is fleeting enjoy your new found happiness.