No this is not true. A person is neither forbidden from marrying the widow of his brother nor is he obligated to marry her, regardless of the presence of children. Rather it is *permissible* - they may marry if they want to and are neither forbidden nor encouraged to do so.

> The wife of one’s brother is not unmarriageable to him forever; rather, as soon as the marriage between her and her husband ends through divorce or death and her ‘Iddah (waiting period) lapses, it becomes **permissible** for the brother of her ex-husband to marry her.
>
> <sup>— [islamweb](https://www.islamweb.net/en/fatwa/125211/marrying-a-deceased-husbands-brother) </sup>

----

- The only people a person is forbidden to marry are his\her mahrams. See https://islam.stackexchange.com/questions/1307/who-are-the-women-that-a-man-is-forbidden-to-marry  The relations of the husband who are forbidden to be married by the wife after his death are the husbands's father and children - not brothers:


  > ولا تنكحوا ما نكح آباؤكم من النساء  ... وحلائل أبنائكم الذين من أصلابكم
  >
  > And do not marry those [women] whom your fathers married ... and [also prohibited are] the wives of your sons who are from your [own] loins
  >
  > <sup>— [Quran 4:22-23](http://quran.com/4/22-24)</sup>
  >
  >وقل للمؤمنات ... لا يبدين زينتهن إلا ... آباء بعولتهن ... أو أبناء بعولتهن 
  >
  > 
  >And tell the believing women to ...  not expose their adornment except to ... their husbands' fathers ... their husbands' sons
  >
  > <sup>— [Quran 24:31](http://quran.com/24/31)</sup>

- There isn't any obligation on a person to marry someone specific. Rather it is permitted to marry whomever one likes among the permitted women:

  > فانكحوا ما طاب لكم من النساء 
  >
  > then marry those that please you of [other] women
  >
  > <sup>— [Quran 4:3](http://quran.com/4/3)</sup>

- The widow is free to marry anyone or even to remain unmarried after the death of her husband. There is no restriction upon her, except for observing the mourning period:
 
  >  فلا جناح عليكم فيما فعلن في أنفسهن بالمعروف
  >
  > then there is no blame upon you for what they do with themselves in an acceptable manner. 
  >
  > <sup>— [Quran 2:234](http://quran.com/2/234)</sup>

- A person does not have the first right to his deceased brother's widows. That was an ancient Arabian custom that the Quran forbade:

  >  لا يحل لكم أن ترثوا النساء كرها 
  >
  > it is not lawful for you to inherit women by compulsion
  >
  > <sup>— [Quran 4:19](http://quran.com/4/19)</sup>
  >
  > كانوا إذا مات الرجل كان أولياؤه أحق بامرأته: إن شاء بعضهم تزوجها، وإن شاءوا زوجها، وإن شاءوا لم يزوجها، فهم أحق بها من أهلها، فنزلت هذه الآية في ذلك
  >
  >  The custom (in the Pre-lslamic Period) was that if a man died, his
  > relatives used to have the right to inherit his wife, and if one of
  > them wished, he could marry her, or they could marry her to somebody
  > else, or prevent her from marrying if they wished, for they had more
  > right to dispose of her than her own relatives. Therefore this Verse
  > was revealed concerning this matter.
  >
  > <sup>— [Bukhari](https://sunnah.com/bukhari/89/9)</sup>