Hurray, excellent news !! You will be able to enjoy the summer in Dorset. There is a steam fair at the end of August very close to Blandford. We went to it for the first time last year and had a fantastic time, even DD who is not the slightest bit into steam engines. It is much bigger than you can possibly imagine until you see it, well worth going to.
If your DD is into arts and crafts there are classes going on at Walford Mill where they make some really lovely stuff.
Moors Valley is a must, they have lots of activities throughout the summer, we're off to something there this weekend.
Badbury Rings on the Blandford to Wimborne Road is good for walks and rolling down the rings, DD that is, not you !
The best ice cream in the world is at Barford Farm, you can sit in their gorgeous gardens and have a cone. Not far down the road is a place where you paddle in the Stour and walk down the river bank.
New Forest on the doorstep, the Fighting Cocks I think it is at Godshill is great to sit outside and watch all the ponies but many more places there.
Crab fishing at Mudeford followed by trip over by boat to Hengistbury Head to the beach and cafe.
Honeybrook Farm, good little farm shop with really good playground and walks through the fields.
Obviously the beaches, many to chose from. Lots of stuff goes on at the summer at Corfe Castle. Fab pub just beyond Corfe, turn right and go up a hill, the Scotsman Arms. Brilliant view over Purbeck.
Hope that gives you a few ideas to look forward to just off the top of my head, there are plenty more !
Not so good about your BIL, hope that gets sorted soon.