I was due to take a ferry to Bruges tomorrow for a murder mystery night and then on to a xmas market on Wednesday. Me and my friend have decided not to go, the main reasons are as follows
Firstly although Brussels and Bruges are not the same place they are in the same place, and I feel that with Brussels itself being on full lock down, schools, concerts etc closed it is more likely that they will move on to smaller less guarded places in the same country (I am aware that it is impossible to second guess these people or their plans, it's more that if j feel like they'd do that then that's what I'm going to be expecting the whole time I'm there, thus ruining my trip!)
Secondly the current terror alert is critical and imminent and they have gone on record saying they have very reliable information that there will be a multi location attack like in Paris!
Thirdly I know you're not travelling on a ferry but it has also been highlighted in the news that ferries are a massive target for terror attacks due to the high concentration of people in a relatively small area and the much lower security on them in comparison with planes etc
This is just my opinion and I understand everyone will judge things differently but I myself know I will not enjoy my time there and therefore there is no point in me going, I am going to try for a refund but do not hold out much hope as they have not issued a no travel warning.
All that being said it's a while until you go and things could easily and quickly change for the better or the worst. Although I understand your predicament with the other family
I imagine the chances of something happening while you are there is quite low, but not impossible! So my advice would be not to try and second guess if and when something might happen, but think about whether you're actually going to enjoy your time there or whether you're going to be worried and panicking the whole time spending the whole time hiding in door for fear of going somewhere crowded or busy, if the latter is the case then in my eyes there's no point in going, holidays are supposed to be relaxing and enjoyable times and you're better off at home if you're not going to get the most out of it! X