This could have been me posting 20 years ago. I was in a practically identical situation. Not only that, my in-laws used to come and park themselves in my house whenever they felt like it, then want food and drinks making, TV on all the time etc.
What I found helped was to just drag myself up early with the dc, put the baby in the pram and head for the park. I walked everywhere, and DS1 walked too.
We took picnics, a ball, anything to encourage DS1 to run around. We could go to the same park every day - when they are little they don't get bored with it.
We would get home for lunch time and usually, if we had walked enough miles, DS1 would sleep for at least an hour, and I would get into bed with DS2, feed him and we would all sleep for an hour.
In the afternoon I would repeat the process.
I lost 2 stone because I walked about 10 miles every day. If we didn't get out and walk every day, DS1 would not sleep. At first I was shattered, but the longer we did it, the easier it got.
Being outside in the sunshine and the fresh air is the best thing you can do. It makes the DC much calmer and better tempered.
Hang in there it does get better.