Hi Danni - I agree with the othe girls, it is your body and baby, not your SIL, so tell you where to go (possible where the sun doesn't shine??)
I have it a little different to the others, whereby I am restricted to activity from the word go.
(I wont go into detail, but have had alot of trouble with all past pregnancies with going into preterm labour, and bleeding etc) But that is just me, and it has been restricted on my obgyn's orders.
I can still exercise, but only the gentler options like walking, swimming etc without problems, but kicking the ball around the backyard with the kids, and running about, have all had to slow right down , as I get pain, and get really short of breath already.
As long as it is your regular excercise, and you feel comfortable, making sure you are well hydarated regularly, there should be no problem. However, I would really think hard about wether you are willing to risk anything that is body contact.
You know what you are comfortable with in your heart, so dont take your SIL too much to heart hun.