Hmm, it's not nice to be deliberately left out, obviously, but it's hard to say whether that's what's going on here or not.
Firstly, you haven't been out with them in 2 years. Maybe they didn't see any reason why this time would be different. Maybe they are waiting for you to say you're ready to meet up with people again and don't want to pressure you after a difficult time. They might have thought it was insensitive to make the offer knowing that you always refuse, or just have got a bit fed up offering if you always say no. This could be a bit nasty on their part or it could be completely benign, it depends on the previous history you've had with them.
Secondly, you haven't said what the actual relationship is between you and the others who are going out. Maybe they're all quite closely related (e.g. a group of sisters) and you're a bit more distant. And they can't invite everyone in the whole extended family. Or maybe they get on with each other better - not to deliberately exclusde you, but e.g. I am closer friends with some of my cousins than others, I don't invite every cousin every time I meet up with some of them just because we all have the same level of family tie.
Anyway, there's a chance it was an oversight, or a misunderstanding because you have a young baby and they didn't realise you would be able to come. If the relationship is generally very good, then it might not hurt to ask them if you would be able to join them this time. But tread carefully with that, depending on the relationship, you don't want to put them in a corner of having to invite you. Otherwise, you could take the initative and organise something yourself, that way they will know that you're up and about again after your two year absence, and will be more likely to invite you to things in the future.