Should I be concerned

I have been married to my husband for four years, on occasions his sister stays when she is off work, the reason for this post is I am concerned about something I saw, in our house the rule is the last one to use the
bathroom cleans the shower cubical, on this particular morning I was my husband, I was about to go out but I had to ask my husband something, I went in to see him what I saw was him naked cleaning the cubical he had a full erection this is not unusual for him in a morning, but his sister was in there with him sat on the toilet lid talking they stopped when they saw me her dressing gown wasn't fastened she was naked as well,
I asked him what he wanted, but I left right away I was embarrassed, should I be worried, are they having sex I know it can happen between brother and sister, or have I got a dirty mind they seemed very comfortable, I don't have a brother so I don't know how close the relationship can be how common is there nakedness,
Comments please

It could have been totally innocent but to be honest the fact he had an erection would suggest he was turned on by her. You didn't say how old they are but having an erection while cleaning the shower does not sound like a turn on!!!! Her being naked could be.
I don't know how many other people have had sex with their bro or sis but I know when I was 16 and alway horny I did sex things with my sis who was a year younger. We never had intercourse but she did suck my dick and give me hand jobs and I fingered her a lot and licked her pussy.

You will understand when it happens its very special and exciting when brother and sister commit the ultimate taboo, its most exciting because its very wrong that what makes it so excruciatingly pleasurable there is no other sexual experience like It,

To the second reply
There is another ultimate taboo than brother and sister

There can only one reason why a brother gets an erection for his sister, lets not pretend we all know what he wants to do to her, It happened to me when I use to go swimming with my sister, we got changed in the same cabin she ignored my erection at first, the second time she took hold of it and gave me a hand job, after we had a full sexual relationship, I don't have any regrets.

Its not only boys who get excited for there sister, girls get sexually excited for there brother, me for one