A few people on here have asked how long is the OP expected to 'pretend' her DS doesn't exist with her DSis. My answer to that is however long it takes to have a coffee and talk this through.
OP's family are saying, rightly or wrongly, that the ball is in OP's court. Yes, her sister could have sent a card, yes she could have sent a gift. Yes, she could have got a friend to do that for her if she couldn't face baby cards and baby clothes. But she didn't. So the choice is either throw in the towel and never be in the same room as her again OP or try to heal the hurt (even though it isn't your fault. It's not hers either - it's just bad luck). Someone has to pull their big sisterly boots up and make the first move.
Before we found out about our unexplained infertility I and two other women in my department were trying to conceive. One got pregnant two years before me, the other a year before me. I kept a brave face on it. They were (still are) my friends. I was pleased for them and I joined in all the maternity leave celebrations, sent a card and gift when they had their babies. But the tears at home were fierce. Everything for us revolved around getting pregnant.
I recently came across photos of me and DH taken at a wedding around the time we were going for tests. We looked so tense. I remember being at that wedding having a great time on the surface but feeling so tired, so unhappy and so 'out of it' like a glass barrier was around me. I put it around me to protect myself but it stayed up for so long that I sometimes found it hard to be a 'normal' me anymore. I can't think of any other way of describing it so my apologies if that description is a bit odd.
Now I'm someone who wasn't bothered about having children. I met my DH late and would have been perfectly happy if we'd made the decision not to bother trying for children. But we did decide to try for a family and as soon as we did I became obsessed. Ovulation charts, different TTC methods, every bloody google page I could read.
I would compare myself to my pregnant colleagues a lot. They were similar in age and they could do get pregnant (in fact three times in one case and twice in the other). I couldn't. Then we went through two lots of IVF. First one of those I came out with no egg retrieval at all. Every time my period came round I felt a little more diminished. I wasn't, of course I wasn't. I'm the same person now as I've always been but my mind just kept saying 'you can't manage this. This is down to you. You're the one who can't get pregnant' and it eats you up. Every day. My DH said he felt exactly the same about himself.
Some people can hide those feelings, some can't. Some can get through it. Some find it very hard to cope. If we were all the same life would be easy. But it's not, it's a minefield sometimes and a field of daisies the next.
The point is that if you sit and talk OP, without baby, and the situation doesn't change you can say, hand on heart, that you have tried. You CAN then say 'how long do I have to pretend DS doesn't exist' - because you have tried.
At the moment there's a wall between the two of you. Someone has to put a door in that wall. She won't so it's down to you and if the door is slammed in your face you'll probably find your family will begin to back you more because they've seen you reaching out. You can't do more than that. But you CAN do that. Just once. Because someone has to.