Msida I'm sorry to hear of the death of your husband last August. It must have been a horrible time for you and you will no doubt still be feeling very emotionally raw. I'm also sorry to hear that you are feeling so alone and unhappy at the moment.
I wondered if it is something new that you are not seeing much of your children and grandchildren. You did mention there was some tension with your daughter in law so perhaps that is part of the problem.
At the moment, I think many people are feeling quite lost and alone because life is so strange. Even people with partners are feeling the strain (and it is quite possible to feel lonely even when there is someone else in the house).
I don't know your family's situation but it could be that work pressures are why they have suggested you visit them. Some people are having to work from home, as well as trying to look after their children and supervise their learning. Some people have to travel to work and are anxious about coming into contact with people. There may also be other pressures relating to Covid, such as decreased finances or anxieties about children's welfare, etc. This can make people feel tired and depressed. It has also been widely reported that there is growing tension in many families, with children becoming more prone to challenging behaviour and angry outbursts. Any of these issues can make it more difficult to focus on friends and relatives outside the home.
You haven't been prevented from going to see them so if you can overcome the discomfort you feel about your daughter in law, perhaps you could make a short visit to the family, just to keep contact going, and hope that things improve gradually.
I hope that some of the posts here have helped a bit and that soon things won't feel so bleak for you. I wish you all the best.