My sister in Law (my wifes elder sister), lost her husband earlier by an accident, after few months her 3 year kid very attached to me. We have no kids at that time. Because we just 1 yr after marriage. So her family and our family decided to live her with me. After 2 weeks we decided to marry and we married. Now we all together living, they both (wife and her sister) love each other alot and we do sex together too. Now i have a kid from my first wife. Her sister want a kid from me. So is there any problem to make her pregnent, my wife is ok with that. (I am 28 yr, my wife 24 and her sister 28yr)
sounds like you have a very open relationship and you all sound realy close, i mean it's not normal at all!! BUT if your wife is ok with it and it will make you all happy then i say go for it! just make sure you think about it properly and make sure when both kids grow up (your child with your wife, and child with her sis) they wont suffer for it i.e bullying, people asking questions etc.
If you are a non- muslim I have no comments to offer. Because I have little knowledge in that situation. But if you are a muslim you cannot marry REAL sister of your wife.A REAL sister of wife can be taken in nikah only in a situation, when the wife is not alive or divorced( divorce case is highly unlikely in the given situation).. In other words two real sisters cannot be taken as wives simultaneously, at the same time. Only one of them can remain your wife, if you are a muslim..( in no case two alive sisters remain married as per nikaah with a muslim man.... it is not allowed at all)... However if you have nothing to do with any religious injunctions than O.K. free to do anything as a liberal individual.... I guess you are not a muslim so you may not face any problem...