Well…I don’t know her, but…from what you’ve said OP: I can’t see very much wrong with her, apart from the fact that for whatever reason, she doesn’t feel ‘at home’ in her husband’s family. That is almost certainly NOT your fault. Some people get on, some people don’t.
When I met my husband, his parents couldn’t have been more welcoming, they invited me for dinner, took me on trips, bought my wedding dress and in EVERY way they’ve been WONDERFUL! … They also know that I feel uncomfortable with phone-calls (I grew up Mennonite, I don’t think I’ll ever get used to new-fangled stuff!), so…she asked me and I said to my MIL: ‘Call at exactly the same time once a week, so I’m ready for it!’. And she does, and we talk for 45 minutes or more! But, she KNOWS I have a social problem, I don’t like being around lots of people, I need to be ‘alone’ a lot…she also knows I make her son happy. And she accepts me for who I am because , and she said that the last time they came up for a week, I ‘am a good mother and wife’. MY MIL and I are very different, and I’m sure she struggles to understand why her son fell in love with me 10 years ago…but I call her ‘mom’ (my own parents died long ago) and when I see my FIL, he’s the only person apart from my husband who’s allowed to hug me! They are very dear to me, but I will admit I need ‘distance’ even with them. It’s how I am, I won’t bore you with my childhood but a lot stems from that…Now…I don’t know your son or DIL, but I DO agree: where is HE in all of this? Why can’t he pick up the phone? Why can’t he keep the lines of communication open? Because I tell my husband every so often ‘Come on, you haven’t spoken to your parents in 3 weeks, go call them!’ I tell him 'They won’t live forever! And when they visit (about 4 x a year for 5-7 days at a time) I encourage him to spend loads of time with his dad in particular. With his mom he has a very ‘complex’ relationship-there is ‘love’, but also a lot of unresolved issues that SHE has buried but that still make it hard for my husband to fully love her and …I guess…trust her.
Now, I don’t know if there is something in your family where your son feels ‘left out’ or just not ‘as close’ as your other children. Perhaps some of the distance is not just due to your DIL, but also to HIM. Again, I’m just looking at my husband here…To me, he’s got great parents, and if you met them you’d love them! They’re wonderful, generous people! But…they do admit he was not ‘planned’, there was a large gap between him and his elder siblings, he never ‘fitted in’ with the rest of the family and with the benefit of hindsight my FIL says they loved him, but…their son felt somehow ‘unwanted’, an ‘afterthought-baby’…and it led to a lot of problems. I tell BOTH sides that you cannot go back and change things, life happens-and I hear from both sides ‘But I/we love him/them’, but there is this unspoken ‘wall’ that keeps them from being as close as they could be. If both sides could just sit down and be 100% honest. But my husband doesn’t want to hurt their feelings by saying ‘I felt unloved’, and his parents say to ME, but not to him ‘Yes, we could have handled a lot of things differently’
Again, all we can do on here is ‘guess’ what might be true in your case, but I’d say: You are NOT doing anything wrong , but neither is your DIL-she’s just very different from you. And your son, really ought to be able to keep the lines of communication with his family open by himself. It wouldn’t hurt to call or write himself.
As for the condolance card: it’s a cultural thing, here in Britain you send it asap, but in Holland where I grew up, you’d turn up for the funeral (if you lived closeby) and send the card a week or two later (with the thought of ‘The pain is not so raw now, they’ll have time to read it now, life will have gotten back on track’). So…give her the benefit of the doubt: she may simply have thought "Well, my husband is at the funeral, giving his condolances, so let’s wait until things are back on track and then send a card a little later’.
Sending fruit-baskets, is a very unfamiliar thought to me, so again-it might be a local/cultural thing that your DIL doesn’t understand, and your son didn’t remember as he’d just come off a flight (and as it was his grandmother, I venture he might have been so upset he simply forgot!)