My Ds is 2.5 and Dnephew is 5 and pretty much every time we all meet up he tries to hurt him or is just plain mean. He has aggression problems and has a social worker because of quite a serious incident at nursery (don't want to give details incase I out myself). My Ds is not a pfb or anything and this is not just boys rough and tumble or kids arguing/bickering which happens - its quite harsh stuff e.g being threatened with a knife, bit, hit, made to eat dirt, shoved into nettles, tried to shove a clothes pole in his eye as well as usual stuff 
Now while i love my nephew I just don't feel my Ds is genuinely safe around him and my gut instinct is to remove ourselves from future family events until DN behaviour can be improved. But then its Ds that will miss out by not seeing his grandparents and cousins and really he shouldn't have to suffer. Dnephew and Sil spend a lot of time at at other cousins and ILs so he just wouldn't get to see them much or do any of the fun outings if we avoid-am totally torn.
Whenever Dnephews behavior gets mentioned it usually causes a huge row (to the point DH is a bit alienated from his family now) and I come away feeling like a bodyguard and drained. WWYD? Any Ideas/experience? am at my wits end as I don't live near my family so its all he's got.
just to point out it's not just Ds he hurts, it's other random people at softplay/parks etc