P2-B1:
SOU! YOU WANT TO GO ON A DATE?!
You tend to leave out words at times. Also, I think Sumire is not stating a fact there, it's more like asking for confirmation, so there should be a question mark at least - the exclamation mark would be optional (if she's really surprised)
P3:
IT'S JUST THAT I FEEL BORED TO STAY AT HOME ALL THE TIME. SINCE YOU ARE FREE RIGHT NOW, NEE-SAN, I THOUGHT IT WOULD BE FINE IF WE GO OUTSIDE AND HAVE A LITTLE FUN.
I rewrote that bubble, no offense. But I think this is what you wanted Sou to say. The blue bits are optional, if you have to keep the bubble size in mind, leave them out as you see fit.
P4:
SOU, WE ARE GOING ON A DATE!!
SO IT'S A DATE WITH SOU!!
I have a hard time deciphering if you mean "Sou," or "So,"... I'll assume Sumire addresses her brother here. Otherwise, maybe use the alternative... again, the blue part is optional.
P5-B1:
I would replace the second "Sou" with "You" here. In Japanese it's fine to use the name, but in English it sounds weird.
P5-B2:
it's NEE-SAN <- you tend to forget the second e quite often.
P6-B1:
W-WHERE ARE WE GOING?
not sure what exactly you want to convey here, but Sumire getting taken by a surprise compliment is probably it. So, it that case, maybe write it like that. Perhaps adjust the fontsize a bit to make it fit the bubble.
P6-B2:
TO THE COFFEE-SHOP NEAR OUR HOUSE?