Found this in Post Secret. I cannot comprehend it. What does it mean?

I'm having my third child. I feel guilty, because my brother-in-law and his wife are infertile.  In other words, she's saying that she feels bad that she's able to have children while other people can't have children. 

Someone is feeling guilty because they are on their 3rd pregnancy, while their brother-in-law and his wife are unable to become pregnant.

The poster is pregnant with their third child and they feel guilty (guilty in this case meaning "sad for" or "bad for") because their in-laws are infertile (meaning the in-laws are incapable of becoming pregnant)

Isn't the wife of her brother in law her sister?

She’s guilty that she’s about to have her third child, because her brother in law and his wife cannot have any children.

OP, you have to help people out and say what part you are having difficulty with. Is it about the couple being "infertile"? Is it the "guilt"? Do you understand what "feeling guilty" means, but you don't know why a woman would feel guilty about having kids just because someone else can't? (I mean, agree.... she shouldn't feel guilty. If that's what you're confused about, I'm confused too -- it's not her job to make everyone in the world healthy and happy. She should be happy for herself).

I'm so stupid. I thought the post was implying OP was sleeping his brother's wife and all three children were his because his brother was infertile.

The person who posted this is feeling guilty because they are having a third child while their sibling is unable to have children of their own (infertile).

Because her sister is infertize but she and her husband want a child, so the girl has to bear a child for them (possibly seeded by her brother-in-law)

I find the wording strange, but it also packs a lot of information in very few words. I believe she’s talking about her husband’s brother and his family, if she were talking about her sister I would think she would have said “my sister and her husband.” It also says that they’re both infertile, which isn’t something we commonly hear when talking about a couple’s ability to have a child. An individual is generally described as infertile, not a couple. However, one could describe a couple as “struggling with infertility”.

I'm guessing the confusion comes from the word "guilty"? It doesn't mean she feels responsible for other people's infertility. She's just sad that people she loves can't have something she has.