Is there only one group for ages 5-7? If so, that's the one your older child is already in, correct? So your younger child would be with her sibling.
If there is more than one group, I'd put your younger child into the age 5-7 group that did NOT include her older sibling. If there is just one group for the 5-7 age -- I would find another activity altogether for your younger child. Take her to the museum yourself during the time your older child is in the group program; museums have guides and tours and "backpacks" full of activities for young kids; do it with her! Or find a different class for her somewhere else.
I have seen so many times how an older child ends up with the younger child tagging along in the same activity. It denies the older child the chance to be on his or her own. Your older child (girl or boy?) will end up resenting the fact that little sister is there; he will feel like the group is now for "little kids." And if your younger child will want to talk to, walk around with and generally get the attention of your older child while they are in the same activity -- that won't work; your older child will soon be telling you "It's boring now." Don't do that to your older child. Let him or her be with peers without a younger sibling there. They have years and years to do things together, and this is already established as something your older child does on his or her own.
Also, while your little girl sounds very mature (which is great, my own kid would have been like yours at that age--ready for bigger stuff), please know that there is a lot of difference in behavior, stamina and maturity between "just turned four" and a child who is seven. If she gets tired faster than the oldest kids in the group, or her attention wanders a little, the leaders will have to focus on her more and the older kids will notice it and resent it. They (and possibly their parents too) will ask why a child clearly a full year below the minimum age is in this group.
I do think it's excellent that the program pays enough attention that adults noticed she is ready for bigger challenges! But I would not put her with an older sibling - it's not really fair to the sibling. If there is a second group for ages 5-7 without the sibling, yes, try it, but only on a trial basis for maybe two sessions. Tag along yourself but stay in the background a much as possible. If the program says you can't tag along a few times to observe her with the older group -- then it's not for her..