DP and I have quite a toxic relationship with DP’s brother (BIL) and his wife (SIL). It’s really getting us down so any constructive advice would be greatly appreciated. Sorry this is so long! I just wanted to make sure I gave the full picture.
First, when we are physically with them (nearly always at wider family events, e.g. Christmas, weddings, birthday parties etc.) they act in a number of hurtful ways. We always make sure we show an interest in their lives/jobs/holidays etc but they will never ask us anything at all about ourselves and will make digs if other people ask us questions. An example is when they had been talking about a holiday they’d been on recently and we’d all said how nice it sounded and then someone said “Conker, aren’t you off on a holiday soon, where’s that to again?” Before I answered, BIL and SIL rolled their eyes to each other and SIL said something like “oh yes of course, everyone’s far more interested in Conker’s holiday plans than ours”. Leaving me to feel very awkward and not sure whether I should answer or not or whether I should say something like “oh no, yours sounds much more interesting, tell us more about that”. But this sounds ridiculous as we’d already heard about their trip and the other relative was just making normal conversation?
They can’t bear it when conversation turns to a topic about anyone other than themselves (unless it’s to slate somebody else, including general groups such as “stupid people” and “poor people” 
!) and I doubt they know anything at all about my job, my family or my friends despite knowing me now for a couple of years. This is hurtful as it seems like they have no interest in me or in getting to know me, despite me making a big effort with them and always being very friendly and open.
On a related note, which is more upsetting, they can never, ever be happy for us. So if either one of us is being congratulated for a new job/new house/sports achievement etc, they will just scowl and not add their own congratulations or even pretend to be happy for us. It really puts a downer on things, especially for DP who would love for his brother to be happy for him and to wish him well. Every so often if in their eyes we’ve had too much attention from other relatives, they will go off on a monologue that on the face of it is very “general” but is actually a pointed dig. So for example, say one of us has just got a new job and everyone else is saying well done, one of them might launch into a bit of a diatribe about how sad it is that some people just focus on work these days and how it means that such people aren’t good friends or relatives as they don’t have enough time for the people that matter to them. (DP and I actually greatly value friends and family and make big effort with both, but the insinuation is that we don’t).
This has led to us changing our behaviour when we’re around them, trying to downplay everything we do and never starting a conversation about anything we’ve done or have coming up, as it’s not worth the grief we’ll get for it. It also makes both of us quite stressed as we’re constantly trying to watch what we say and if we ‘slip up’ and talk about ourselves, we are ‘rewarded’ with digs followed by an unpleasant atmosphere. They also launch into these speeches and make digs at MIL and FIL so it’s not just us, but we do feel like the main targets.
Other family members either don’t seem to notice or pretend not to – nobody else wants any drama so MIL will briskly try to say something upbeat or FIL will start a new random and banal topic of conversation like the weather just to change the subject! But the bad atmosphere still lingers 
We only see them 4-5 times a year but we dread each occasion for a couple of weeks leading up to it, don’t enjoy the occasion itself, which is a real shame when it’s something like a wedding or Christmas and then spend the next few days feeling low about the event itself and about the relationship in general.
When we’re not physically with them they act like we don’t exist. So they never initiate any kind of contact with us, never ask us to meet up (even though neither of us have DC and we live 20 mins away) and never interact with us on social media. I wouldn’t mind this at all if they were just the type to keep themselves to themselves (and in fact we welcome the break from them!), but they are actually quite social and often arrange to meet up with DP’s other sibling and their partner and with various other family members (DP’s parents, DP’s cousins, DP’s aunts and uncles etc), plus they are very active on social media and are always talking about how important “family” is to them on there, but apparently we are not included in this “family”. This is hurtful as it feels like a personal snub. We have invited them round and do initiate messages occasionally but always end up regretting it as they come back with something negative or having a dig at us or someone else.
Last weekend things turned a bit of a corner for us as they were organising a big family event and we saw both SIL and BIL be quite rude to numerous other people (before it had always felt like it was mainly just targeted at us) and SIL was in a big huff about a group of people who apparently hadn’t helped enough or in the right way, but I’d seen these people offer plenty of help and generally be great guests – they hadn’t acted wrongly at all. It made us realise that actually BIL and SIL are both just quite negative people and they seem to dislike or have issues with lots of people, not just with us. It has helped a little to know that I perhaps don’t need to take their behaviour so personally in future as it’s more about their general nature than about their feelings towards us specifically, but it has also made us quite sad to realise that actually they are very unlikely to ever change, as it’s not about something we’ve done/are doing (although I’d always known I hadn’t done anything to upset them and have in fact always been very friendly and kind to them – it was more that they seemed to hate my very existence for some reason!). They do seem to have a particular problem with us though that appears to stem from insecurity, even though they have plenty to celebrate in their own lives (have got married recently, bought a lovely house, doing well in their jobs) - they seem to be living in constant competition with us, then judging themselves to have done worse and being annoyed at us for that(!), even though we have no interest in competing and are pleased for them whenever they have good news.
Has anyone come across anyone like this before and does anyone have any helpful tactics for dealing with them? At the moment we are fairly low contact but are planning to go lower by not initiating any contact. We would still have the 4-5 wider family events a year to deal with though and we would like to enjoy these if possible! I’ve really wanted them to like me and didn’t understand why they didn’t as I make an effort with them and genuinely care about them being happy but I’m starting to realise that I’m not actually doing anything wrong so I shouldn’t be changing who I am for them.
If I came across people like this in any other context I would just avoid them like the plague but as they’re family it’s not so simple. I don’t think “having it out” with them would help at all as it’s just their nature and it would also really upset lovely MIL and FIL if there was a big fall out where people were going NC.
Does anyone have any constructive advice for dealing with them? (and sorry again that this is so long!)