just saw the post and though will share some of my thoughts, i'm a mom of a single child. my son is 6 years old and me and my dh thinks we will stop with him,
i have an older sis and till today we do not have any so called special bonding as sisters, it is more of a duty, that we do things. even my dh has a younger sis who shows no signs of affection, although my dh is a type of guy who is very emotional and loving by nature.
we feel, when we survive without any sibling love or comfort, even our son can when he grows up, we pray that he gets a great girl as his wife, because that relationship will be important more than any other (that is the world truth), and just because he is our only kid we don't pamper him, we encourage him all the time at the same time put a stop when he has to be corrected.
i have enrolled my son in swimming lessons and so far he is doing great, he is already in level 6, he is also good in math and reading, he is an amazing and loving kid, he hugs and tells us that he loves us all the time, i don't think any sibling is going to bring out any other quality in him, we encourage him to share stuff and be nice with other kids, infact when we meet our friends little ones, he takes care of them as his own,
And after reading all the encouraging posts i have come to a conclusion that we have made a right decision

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thanks again,
radha